Apolar Segments Research Articles

Transient water wires mediate selective proton transport in designed channel proteins.

Selective proton transport through proteins is essential for forming and using proton gradients in cells. Protons are conducted along hydrogen-bonded 'wires' of water molecules and polar side chains, which, somewhat surprisingly, are often interrupted by dry apolar stretches in the conduction pathways, inferred from static protein structures. Here we hypothesize that protons are conducted through such dry spots by forming transient water wires, often highly correlated with the presence of the excess protons in the water wire. To test this hypothesis, we performed molecular dynamics simulations to design transmembrane channels with stable water pockets interspersed by apolar segments capable of forming flickering water wires. The minimalist designed channels conduct protons at rates similar to viral proton channels, and they are at least 106-fold more selective for H+ over Na+. These studies inform the mechanisms of biological proton conduction and the principles for engineering proton-conductive materials.

Post-translational acylation controls the folding and functions of the CyaA RTX toxin.

The adenylate cyclase (CyaA) toxin is a major virulence factor of Bordetella pertussis, the causative agent of whooping cough. CyaA is synthetized as a pro-toxin, pro-CyaA, and converted into its cytotoxic form upon acylation of two lysines. After secretion, CyaA invades eukaryotic cells and produces cAMP, leading to host defense subversion. To gain further insights into the effect of acylation, we compared the functional and structural properties of pro-CyaA and CyaA proteins. HDX-MS results show that the refolding process of both proteins upon progressive urea removal is initiated by calcium binding to the C-terminal RTX domain. We further identified a critical hydrophobic segment, distal from the acylation region, that folds at higher urea concentration in CyaA than in pro-CyaA. Once refolded into monomers, CyaA is more compact and stable than pro-CyaA, due to a complex set of interactions between domains. Our HDX-MS data provide direct evidence that the presence of acyl chains in CyaA induces a significant stabilization of the apolar segments of the hydrophobic domain and of most of the acylation region. We propose a refolding model dependent on calcium and driven by local and distal acylation-dependent interactions within CyaA. Therefore, CyaA acylation is not only critical for cell intoxication, but also for protein refolding into its active conformation. Our data shed light on the complex relationship between post-translational modifications, structural disorder and protein folding. Coupling calcium-binding and acylation-driven folding is likely pertinent for other repeat-in-toxin cytolysins produced by many Gram-negative bacterial pathogens.-O'Brien, D. P., Cannella, S. E., Voegele, A., Raoux-Barbot, D., Davi, M., Douché, T., Matondo, M., Brier, S., Ladant, D., Chenal, A. Post-translational acylation controls the folding and functions of the CyaA RTX toxin.

Diverse Role of Solvents in Controlling Supramolecular Chirality.

Supramolecular self-assembly stands for the spontaneous aggregation of small organic compounds or polymers into ordered structures at any scale. When being induced by inherent molecular chiral centers or ambient asymmetric factors, asymmetric spatial arrangement between building units shall occur, which is defined as supramolecular chirality. Except for molecular design, utilizing external stimulus factors to tune supramolecular chirality is a promising approach. In this Concept article, we particularly discuss the important role of solvents in manipulating the chirality of self-assembled systems. The impact of solvents on the chirality is generally based on three properties of solvents, i.e., chirality, polarity, and active coassembly with building blocks. Molecular self-assembly in chiral solvents could undergo the chirality transfer, exhibiting a chiral induction effect. Solvent polarity often determines intermolecular orientation. As a consequence, those building blocks with both polar and apolar segments might change their chirality depending on the solvent polarity. We elaborate the active participation of solvent molecules into ordered structures together with building blocks, where solvents and building blocks exhibit a coassembly manner. By specific treatments such as heating and cooling, solvents could be released or re-entrapped, allowing a smart control over supramolecular chirality. The solvent effect in manipulating two-dimensional chiral self-assemblies is then discussed. The perspective and future development in this research field are presented at last.

Polymorphism of cis-1,4-cyclohexanediol, a new plastic crystal former. Considerations on isomeric cyclohexanediols plastic crystal forming abilities

In this work an investigation on the polymorphism of cis-1,4-cyclohexanediol has been undertaken with emphasis on solid forms obtained in heating/cooling cycles, performed between 0°C and 115°C. A multidisciplinary investigation using differential scanning calorimetry, polarized light thermal microscopy and X-ray diffraction analysis was performed. A crystalline structure of the title compound was solved for the first time: an anisotropic solid phase, polymorph II, which gives rise on heating to a plastic crystalline phase, polymorph I, at T=(101.1±0.2)°C, which melts at T=(108.5±0.2)°C. A complex thermal behavior is observed in cooling/heating cycles performed on the molten compound, with evidence of the existence of other polymorphic forms (another ordered one, form III and form IV, tentatively assigned as plastic crystal).A comparative analysis of the crystalline structures of cis-1,4-cyclohexanediol, cis-1,2-cyclohexanediol, both plastic crystal formers, and of trans-1,4-cyclohexanediol, trans-(1S,2S)-cyclohexanediol, which do not present this mesophase, is performed by Hirshfeld surface analysis. Some similarities could be found between the plastic crystal formers which contrast with the results for the trans-isomers, namely in what concerns fingerprint plots (with much higher anisotropy for the latter), asphericity values (quite similar for the plastic crystalline formers with higher values found for the trans-isomers). It is also worth noting that in cis-1,2- and cis-1,4-cyclohexanediol, the H-bond contacts are confined in one side of the Hirshfeld surface, with the apolar segments of the molecule in the opposite side of the surface. This is also observed for trans-(1S,2S)-cyclohexanediol, but for this non-plastic crystal former, close C–H⋯H–C contacts are identified.For this class of compounds Hirshfeld surface analysis gives significant clues for their different plastic crystal forming abilities.

Interactions between Poly(acrylic acid) and Sodium Dodecyl Sulfate: Isothermal Titration Calorimetric and Surfactant Ion-Selective Electrode Studies

Interaction between a monodispersed poly(acrylic acid) (PAA) (M(W) = 5670 g/mol, M(w)/M(n) = 1.02) with sodium dodecyl sulfate (SDS) was investigated using isothermal titration calorimetry (ITC), ion-selective electrode (ISE), and dynamic light scattering measurements. Contrary to previous studies, we report for the first time evidence of interaction between SDS and PAA when the degree of neutralization (alpha) of PAA is lower than 0.2. Hydrocarbon chains of SDS cooperatively bind to apolar segments of PAA driven by hydrophobic interaction. The interaction is both enthalpy and entropy favored (deltaH is negative but deltaS is positive). In 0.05 wt % PAA solution, the SDS concentration corresponding to the onset of binding (i.e., CAC) is approximately 2.4 mM and the saturation concentration (i.e., C(S)) is approximately 13.3 mM when alpha = 0. When PAA was neutralized and ionized, the binding was hindered by the enhanced electrostatic repulsion between negatively charged SDS and PAA chains and improved solubility of the polymer. With increasing alpha to 0.2, CAC increases to approximately 6.2 mM, C(S) drops to 8.6 mM, and the interaction is significantly weakened where the amount of bound SDS on PAA is reduced considerably. The values of CAC and C(S) derived from different techniques are in good agreement. The binding results in the formation of mixed micelles on apolar PAA coils, which then expands and dissociates into single PAA chains. The majority of unneutralized PAA molecules exist as single polymer chains stabilized by bound SDS micelles in solution after the saturation concentration.

Interaction between Polyelectrolyte and Oppositely Charged Surfactant: Effect of Charge Density

The binding of dodecyltrimethylammonium bromide (DoTab) to poly(acrylic acid) (PAA) at different degrees of neutralization (α) was investigated using isothermal titration calorimetry (ITC) and laser light scattering (LLS) techniques. The surfactant binds to the polymer at all α; however, the mechanism varies. When α is lower than a critical value (αC), the hydrocarbon chains of DoTab cooperatively bind to the apolar segments of PAA driven by hydrophobic interaction at very low DoTab concentration (CDoTab ≤ 0.2 mM). In this binding region, the ITC profile exhibits a significant exothermic peak and the mixture precipitates, which is attributed to the interchain complexation via hydrogen bonding induced by the binding. The precipitation is soon resolubilized with further addition of surfactant as more DoTab micelles are bound to the polymer backbones with their ionic headgroups extending outward. When α > αC, the hydrophobic binding ceases as the polymer is progressively ionized and DoTab binds to the charged polymer chains driven by electrostatic attraction. The condensed counterions on the charged polymer chains are released via the ion-exchange process, resulting in an endothermic maximum on the ITC profile. The value of αC determined from ITC is approximately 0.3, which is reasonably close to the theoretical value derived from the Manning's counterion condensation theory (approximately 0.35).

Enthalpy of hydrophobic hydration separately estimated for each CH 2, CF 2, and OCH 2CH 2 unit component of amphiphiles and of related compounds

The enthalpy of hydrophobic hydration, Δ H HH, was separately estimated for each apolar segment of amphiphiles and of related compounds. The specimens analyzed in this study were the homologs and analogs of sodium alkyl sulfate, alkyltrimethylammonium bromide, ammonium perfluoroalkylcarboxylate, and polyethylene glycols. The Δ H HH was calculated from the adiabatic compressibility β of aqueous solution systematically measured for each homologs at varying concentrations and temperatures. Those data reported in the previous paper were utilized. The Δ H HH estimated for each CH 2, CF 2, and OCH 2CH 2 segments in both hydrophobic and hydrophilic parts of the amphiphiles was found to change depending on its locations in the molecule. The Δ H HH for the first CH 2 unit which is adjacent to the head sulfate group of H(CH 2) n SO 4Na was found to be negligibly small; however, it increases gradually as the position of the unit moves apart from the anionic head group and attains as large as −26.5 kcal/mol for the eighth CH 2. The change of Δ H HH of CF 2 unit in F(CF 2) n COONH 4 shows the similar tendency to H(CH 2) n SO 4Na. The value of Δ H HH is as small as −8 kcal/mol for the first CF 2, however, it increases to −16 kcal/mol for the fifth CF 2, which is 1.5 times larger than that of the fifth CH 2 of alkyl sulfate. Contrary to this, all the Δ H HH of the methylenes of cationic surfactant ranging from the second to the eighth CH 2 units of H(CH 2) n N(CH 3) 3Br take a similar value around −8 kcal/mol of CH 2 irrespective of their positions. The first CH 2 unit which is equivalent to that of quaternary ammonium [HCH 2] 4NBr shows slightly higher value of −9.0 kcal/mol of the first CH 2 than those for the second to eighth CH 2. This seems to suggest that the cationic core nitrogen has an enhancing effect on hydrophobic hydration. The hydration enthalpy Δ H OEH of each oxyethylene (OE) unit of polyethylene glycol was also estimated for H(OCH 2CH 2) p OH ( p=1–4) homologues to see the roles of ether oxygen and hydroxyl groups in hydrophobic hydration. The value of Δ H OEH showed its maximum −13.1 kcal/mol for the OE unit located at the middle of three. The other two units at both ends were found to take the value of −9.9 kcal/mol, smaller than that at the center. This could suggest that the hydrophobic hydration of the ethylene segment is largely enhanced by the neighboring ether oxygens.

Hydrophilicity of Polar and Apolar Domains of Amphiphiles

The hydrophilicity of polar and apolar domains of various amphiphiles was systematically estimated for their homologues and analogues by measuring the molar adiabatic compressibility of an aqueous solution at infinite dilution. The homologues of protic alkyl H(CH2)n–, perfluoroalkyl F(CF2)n–, and alkylphenyl H(CH2)n(C6H5)– groups (n=0–10) were chosen to represent apolar hydrophobic domains. The polar hydrophilic domains tested were –SO4Na, –SO3Na, –COONH4, –N(CH3)3Br, N(CmH2m+1)4Br (m=1–5), and –NH(CH2)nSO3 (n=3, 4) groups. Also tested were the tetraphenyl ionic compounds (C6H5)4MX (M=B/X=Na, M=P/X=Cl, M=As/X=Cl) to study the effect of the ionic sign of the core atom across the tetraphenyl apolar shell, the polyethylene glycols H(OCH2CH2)mOH (m=1–4) to study the role of apolar –CH2– units in the hydrophilic oxyethylene group, and the zwitterionic dimethylaminoalkylsulfonate (CH3)2NH(CH2)nSO3 homologues to study the effect of intramolecular salt formation on the hydrophilicity of the zwitterion. The adiabatic compressibility of the solution was calculated from measurement of the sound velocity and density of solutions. The introduction of laboratory automation and the numerical control of the system improved the accuracies and efficiencies of the measurements a great deal. The range of the temperature scan was 0–40°C with an effective accuracy of ±0.001°C and the concentration was automatically scanned down to far below the cmc of the surfactant. The hydrophilicity of various polar and apolar substances was estimated as the decrease of molar adiabatic compressibility of the aqueous solution with increased concentration of their homologues and analogues. The hydrophobic hydration of nonpolar substances was found to be very small at room temperature and was barely detected above 40°C; however, it became large as the temperature was lowered and attained a maximum at 0°C. The cationic charge of quaternary ammonium N+(CnH2n+1)4 was found to enhance the hydrophobic hydration of methylene groups located at a distance of 4 to 6 Å from the core nitrogen atom, while the terminal negative charge of the anionic surfactant R–SO4−, R–SO3−, or R–COO− was found to decrease the hydrophobic hydration of –CH2– units within the same range. The hydrophilicity of quaternary ammonium and the tetraphenyl ions should be synergistically given by both hydrophobic and ionic hydrations. The hydrophilicity of the perfluoromethylene unit –CF2– was found to have a value comparable to that of the protic methylene unit –CH2–. The hydrophobic hydration seems to offer a good measure of the hydrophilicity of apolar substances; however, it does not necessarily represent the “hydrophobicity” of the apolar segment when the “surface activity” of the amphiphile is concerned.

The Topogenic Contribution of Uncharged Amino Acids on Signal Sequence Orientation in the Endoplasmic Reticulum

Signal sequences for insertion of proteins into the endoplasmic reticulum induce translocation of either the C- or the N-terminal sequence across the membrane. The end that is translocated is primarily determined by the flanking charges and the hydrophobic domain of the signal. To characterize the hydrophobic contribution to topogenesis, we have challenged the translocation machinery in vivo in transfected COS cells with model proteins differing exclusively in the apolar segment of the signal. Homo-oligomers of hydrophobic amino acids as different in size and shape as Val(19), Trp(19), and Tyr(22) generated functional signal sequences with similar topologies in the membrane. The longer a homo-oligomeric sequence of a given residue, the more N-terminal translocation was obtained. To determine the topogenic contribution of all uncharged amino acids in the context of a hydrophobic signal sequence, two residues in a generic oligoleucine signal were exchanged for all uncharged amino acids. The resulting scale resembles a hydrophobicity scale with the more hydrophobic residues promoting N-terminal translocation. In addition, the helix breakers glycine and proline showed a position-dependent effect, which raises the possibility of a conformational contribution to topogenesis.

The Role of the Hydrophobic Domain in Orienting Natural Signal Sequences within the ER Membrane

The orientation of signal sequences during insertion into the endoplasmic reticulum membrane is largely determined by the charged residues flanking the apolar domain. Using recombinant and mutant proteins, also length and hydrophobicity of the apolar segment were shown to affect the orientation: translocation of the N-terminus was found to be favored by long hydrophobic sequences, and translocation of the C-terminus, by short ones. Here, we tested the physiological significance of this phenomenon by mutagenesis of the hydrophobic portion of two natural signals with unusual flanking charges. Extending the hydrophobic domain of the short, cleaved Ncyt/Cexosignal of pre-pro-vasopressin-neurophysin II and shortening that of the Nexo/Ccytsignal anchor of microsomal epoxide hydrolase resulted in a significant fraction of polypeptides inserting in the opposite orientation to that of the wild-type proteins. The topogenic contribution of the hydrophobic domain is thus important for the correct and uniform orientation of natural proteins and can explain the behavior of some of the signals with unusual flanking charges.

Charged residues are major determinants of the transmembrane orientation of a signal-anchor sequence.

Uncleaved signal-anchor sequences of membrane proteins inserted into the endoplasmic reticulum initiate the translocation of either the amino-terminal or the carboxyl-terminal polypeptide segment across the bilayer. Which topology is acquired is not determined by the apolar segment of the signal but rather by the hydrophilic sequences flanking it. To study the role of charged residues in determining the membrane topology, the insertion of mutants of the asialoglycoprotein receptor H1, a single-spanning protein with a cytoplasmic amino terminus, was analyzed in transfected COS-7 cells. When the charged amino acids flanking the hydrophobic signal were mutated to residues of opposite charge, half the polypeptides inserted with the inverted orientation. When, in addition, the amino-terminal domain of the mutant protein was truncated, approximately 90% of the polypeptides acquired the inverted topology. The transmembrane orientation appears to be primarily determined by the charges flanking the signal sequence but is modulated by the domains to be translocated.

Modulation of the general anesthetic sensitivity of a protein: a transition between two forms of firefly luciferase.

The activities of most proteins are relatively insensitive to general anesthetics. A notable exception is firefly luciferase, whose sensitivity to a wide range of anesthetic agents closely parallels that of whole animals. We have now found that this sensitivity can be controlled by ATP. The enzyme is insensitive at low (microM) concentrations of ATP and very sensitive at high (mM) concentrations. The differential sensitivity varies from anesthetic to anesthetic, being greatest (about a 100-fold difference) for molecules with large apolar segments. This suggests that anesthetic sensitivity is modulated by changes in the hydrophobicity of the anesthetic-binding pocket. Parallel changes in the binding of the substrate firefly luciferin, for which anesthetics compete, indicate that anesthetics bind at the same site as the luciferin substrate. These changes in the nature of the binding pocket modify not only the sensitivity to anesthetics but also the position of the "cutoff" in the homologous series of primary alcohol anesthetics; the cutoff position can vary from octanol to pentadecanol, depending upon the concentration of ATP. Our results suggest that particularly sensitive anesthetic target sites in the central nervous system may possess anesthetic-binding pockets whose polarities are regulated by neuromodulatory agents.

Secretion and membrane assembly

Cytoplasmic proteins undergo rapid and stable folding which buries their apolar segments. In contrast, precursors of secreted and membrane proteins have apolar segments which are recognized by chaperones and membranes receptors to distinguish them from soluble proteins.

Statistical thermodynamics of association colloids. III. The gel to liquid phase transition of lipid bilayer membranes

A new theory is introduced to model the lipid membrane structure and stability both above and below the gel to liquid phase transition temperature. Recently, we elaborated a self-consistent-field (SCF) theory, in which the full set of conformations was generated in a rotational isomeric state scheme and Boltzmann statistics was used to determine the statistical weight per conformation. In the present paper we also take into account that the anisotropic distribution of the molecules on the lattice induce a self-consistent anisotropic molecular field. This field, which is a function of the bond orientations, leads to an extra factor in the statistical weight of each conformation and is based on a generalization of Flory’s and Di Marzio’s analysis of systems with rigid rods. This elegant refinement follows from elementary statistics, is free of new adjustable parameters, and significantly improves details of the structure of the model membranes. To examine the properties of the SCAF (self-consistent anisotropic field) theory we use a model membrane built up by lecithin-like molecules composed of apolar and polar segments. The model has three nearest-neighbor interaction parameters of the Flory–Huggins type, namely for the interaction between apolar segments and water, that between polar segments and water, and that between polar and apolar segments. A fourth parameter is the dihedral trans/gauche energy difference. The theory predicts a first order gel to liquid phase transition for the model membranes. Depending on the membrane concentration, both an intercalated (in the dilute regime) and a nonintercalated (in the concentrated regime) gel phase are observed. Detailed information on the various membrane phases is obtained. Order parameter and segment density profiles are given.

Insertion of a multispanning membrane protein occurs sequentially and requires only one signal sequence

To study the insertion of multispanning membrane proteins into the endoplasmic reticulum, we constructed novel proteins on the cDNA level by repeating, up to four times, the internal signal-anchor domain of the asialoglycoprotein receptor H1. Upon in vitro translation in the presence of microsomes, these polypeptides are indeed inserted as polytopic membrane proteins. The first hydrophobic domain functions as a signal and the second as a stop-transfer sequence, while the third initiates a second translocation process, halted again by the fourth. We were able to demonstrate that insertion occurs sequentially, starting with the first apolar segment from the amino terminus. By replacing the original signal-anchor domains by a mutant sequence not recognized by signal recognition particle (SRP), it was shown that only the first hydrophobic domain needs to be a signal sequence and that the second translocation event does not require SRP.

Topogenic signals in integral membrane proteins.

Integral membrane proteins are characterized by long apolar segments that cross the lipid bilayer. Polar domains flanking these apolar segments have a more balanced amino acid composition, typical for soluble proteins. We show that the apolar segments from three different kinds of membrane-assembly signals do not differ significantly in amino acid content, but that the inside/outside location of the polar domains correlates strongly with their content of arginyl and lysyl residues, not only for bacterial inner-membrane proteins, but also for eukaryotic.proteins from the endoplasmic reticulum, the plasma membrane, the inner mitochondrial membrane, and the chloroplast thylakoid membrane. A positive-inside rule thus seems to apply universally to all integral membrane proteins, with apolar regions targeting for membrane integration and charged residues providing the topological information.

Quaternary structure of intestinal maltase–glucoamylase in pancreatectomized rats

Detergent-solubilized intestinal maltase-glucoamylase was isolated 1 week postpancreatectomy (dMpanc) and purified in the presence of detergent and protease inhibitors. Upon sodium dodecyl sulfate - polyacrylamide gel electrophoresis under nondissociating conditions, the major band had a molecular weight of 280,000, slightly smaller than similar bands from detergent (dM) and papain (pM) solubilized maltase from nonpancreatectomized rats. Upon octyl-Sepharose CL-4B chromatography, 57% of the enzyme was eluted by aqueous buffer, unlike pM which was almost completely eluted or dM, 95% of which bound to the column. All fractions of dMpanic from octyl-Sepharose 4B were reduced, by boiling +/- beta-mercaptoethanol, to monomeric subunits, indicating that processing by pancreatic enzymes at the level of the brush border is not a requirement for the appearance of subunits in the rat. As well, under these dissociating conditions, the 145,000 subunit previously identified with the apolar terminus was present in all fractions of dMpanc, including the aqueous fraction, whereas pM contained only the 130,000 subunit. The presence of dMpanc in the aqueous fraction cannot be explained, therefore, by proteolytic cleavage of an apolar anchor segment from the 145,000 subunit. Pancreatic enzymes may affect the enzyme in a minor fashion, however, since aqueous solubility was enhanced and the apparent molecular weight was reduced by pancreatectomy, suggesting a more compact conformation with shielding of apolar segments.

Molecular model of the action potential sodium channel.

Secondary and tertiary structural models of sodium channel transmembrane segments were developed from its recently determined primary sequence in Electrophorus electricus. The model has four homologous domains, and each domain has eight homologous transmembrane segments, S1 through S8. Each domain contains three relatively apolar segments (S1, S2 and S3) and two very apolar segments (S5 and S8), all postulated to be transmembrane alpha-helices. S4 segments have positively charged residues, mainly arginines, at every third residue. The model channel lining is formed by four S4 transmembrane alpha-helices and four negatively charged S7 segments. S7 segments are postulated to be short, partially transmembrane amphipathic alpha-helices in three domains and a beta-strand in the last domain. S7 segments are preceded by short apolar segments (S6) postulated to be alpha-helices in three domains and a beta-strand in the last domain. Positively charged side chains of S4 form salt bridges with negatively charged side chains on S7 and near the ends of S1 and S3. Putative extracellular segments that contain 5 of the 10 potential N-glycosylation sites link S5 to S6. Channel activation may involve a 'helical screw' mechanism in which S4 helices rotate around their axes as they move toward the extracellular surface.

The subunit structure of the eukaryotic chromosome.

New strucutral data have been obtained from neutron scattering studies of chromatin. The concentration-dependent meridional peak at 10-11 nm comes from the interparticle spacing of a subunit structure. Peaks at 5.5 and 3.7 nm have a different contrast behaviour to those at 11.0 and 3.7 nm showing that histones and DNA have a different spatial arrangement in the subunit. A globular model in which apolar segments of histones from the core surrounded by DNA complexed with the basic segments of histones agrees with the data.

Chapter 1 Histones, Chromatin Structure, and Control of Cell Division

Publisher Summary This chapter discusses the histones, and outlines the cross interactions of histones and the basic structural unit of chromatin. In addition, the chapter discusses the neutron diffraction of chromatin and the role of histone H1 phosphorylation in the cell cycle. Because of their close association with DNA, histones have been implicated in many aspects of both the structure and the control of the functions of chromosomes. An important function of histones is the maintenance and control of the structure of the chromosomes through the cell cycle. Studies showed that chromatin had a globular subunit structure. A model is proposed for this structure, where DNA, complexed with the basic segments of histones H2A, H2B, H3, and H4, is coiled on the outside of a core formed by interacting the apolar segments. The very lysine-rich histone H1 is located on the outside of this structure. The location of the DNA model makes it available both for the recognition and enzyme attack, and residues in the basic segments of histones are also available for chemical modification. Chemical modifications of histones occur at residues in the basic segments and these modifications control the interactions between the histones and DNA throughout the genome. In this respect, phosphorylation of H1 is of considerable interest. Further, this H1 phosphorylation is controlled by a large and cyclical variation in the net phosphorylating activity of the cell nucleus. The phosphorylation of H1 is involved in the initial steps of chromosome condensation and that the cyclical variation in enzyme activity is an important factor in the control of cell division.