Clusters Of Charged Amino Acids Research Articles

Molecular Identification of a Mechanosensitive Channel in Archaea

The TM1 domain of the large conductance mechanosensitive (MS) channel of Escherichia coli was used as a genetic probe to search the genomic database of the archaeon Methanoccoccus jannashii for MscL homologs. We report that the hypothetical protein MJ0170 of M. jannashii exhibited 38.5% sequence identity with the TM1 domain of Eco-MscL. Moreover, MJ0170 was found to be a conserved homolog of MscS, the second type of E. coli MS channel encoded by the yggB gene. Furthermore, we identified a cluster of charged residues KIKEE in the C-terminus of MJ0170 that strikingly resembled the charged C-terminal amino acid cluster present in Eco-MscL (RKKEE). We cloned and expressed MJ0170 in E. coli, which when reconstituted into liposomes or expressed in the cell membrane of giant E. coli spheroplasts, exhibited similar activity to the bacterial MS channels. Our study suggests that the M. jannashii MS channel and its homologs evolved as a result of gene duplication of the ancestral MscL-like molecule with the TM1 domain remaining the most conserved structural motif among prokaryotic MS channels.

A transmembrane peptide from the human EGF receptor: behaviour of the cytoplasmic juxtamembrane domain in lipid bilayers

Solid state 2H NMR spectroscopy was employed to study peptides related to the transmembrane domain of the human epidermal growth factor receptor, for insight into the interaction of its cytoplasmic juxtamembrane domain with the membrane surface. Since such receptors have clusters of (+)charged amino acids in this region, the effect of (−)charged phosphatidylserine at the concentration found naturally in the cytoplasmic leaflet (15 mol%) was considered. Each peptide contained 34 amino acids, which included the hydrophobic 23 amino acid stretch thought to span the membrane and a ten amino acid segment beyond the ‘cytoplasmic’ surface. Non-perturbing deuterium probe nuclei were located within alanine side chains in intramembranous and extramembranous portions. 2H NMR spectra were recorded at 35°C and 65°C in fluid lipid bilayers consisting of (zwitterionic) 1-palmitoyl-2-oleoylphosphatidylcholine, with and without 15 mol% (anionic) phosphatidylserine. The cationic extramembranous portion of the receptor backbone was found to be highly rotationally mobile on a time scale of 10 −4–10 −5 s in both types of membrane – as was the α-helical intramembranous portion. Deuterium nuclei in alanine side chains (-CD 3) detected modest changes in peptide backbone orientation and/or dynamics related to the presence of 1-stearoyl-2-oleoylphosphatidylserine: in the case of the extramembranous portion of the peptide these seemed related to lipid charge. Temperature effects on the peptide backbone external to the membrane were qualitatively different from effects on the helical transmembrane domain – likely reflecting the different physical constraints on these peptide regions and the greater flexibility of the extramembranous domain. Effects related to lipid charge could be detected in the spectrum of CD 3 groups on the internally mobile side chain of Val 650, six residues beyond the membrane surface.

High Affinity Binding between Lipoprotein Lipase and Lipoproteins Involves Multiple Ionic and Hydrophobic Interactions, Does Not Require Enzyme Activity, and Is Modulated by Glycosaminoglycans

Lipoprotein lipase (LPL) physically associates with lipoproteins and hydrolyzes triglycerides. To characterize the binding of LPL to lipoproteins, we studied the binding of low density lipoproteins (LDL), apolipoprotein (apo) B17, and various apoB-FLAG (DYKDDDDK octapeptide) chimeras to purified LPL. LDL bound to LPL with high affinity (K(d) values of 10(-12) m) similar to that observed for the binding of LDL to its receptors and 1D1, a monoclonal antibody to LDL, and was greater than its affinity for microsomal triglyceride transfer protein. LDL-LPL binding was sensitive to both salt and detergents, indicating the involvement of both hydrophobic and hydrophilic interactions. In contrast, the N-terminal 17% of apoB interacted with LPL mainly via ionic interactions. Binding of various apoB fusion peptides suggested that LPL bound to apoB at multiple sites within apoB17. Tetrahydrolipstatin, a potent enzyme activity inhibitor, had no effect on apoB-LPL binding, indicating that the enzyme activity was not required for apoB binding. LDL-LPL binding was inhibited by monoclonal antibodies that recognize amino acids 380-410 in the C-terminal region of LPL, a region also shown to interact with heparin and LDL receptor-related protein. The LDL-LPL binding was also inhibited by glycosaminoglycans (GAGs); heparin inhibited the interactions by approximately 50% and removal of trace amounts of heparin from LPL preparations increased LDL binding. Thus, we conclude that the high affinity binding between LPL and lipoproteins involves multiple ionic and hydrophobic interactions, does not require enzyme activity and is modulated by GAGs. It is proposed that LPL contains a surface exposed positively charged amino acid cluster that may be important for various physiological interactions of LPL with different biologically important molecules. Moreover, we postulate that by binding to this cluster, GAGs modulate the association between LDL and LPL and the in vivo metabolism of LPL.

Charge-to-alanine mutagenesis of the adeno-associated virus type 2 Rep78/68 proteins yields temperature-sensitive and magnesium-dependent variants.

The adeno-associated virus type 2 (AAV) replication (Rep) proteins Rep78 and 68 (Rep78/68) exhibit a number of biochemical activities required for AAV replication, including specific binding to a 22-bp region of the terminal repeat, site-specific endonuclease activity, and helicase activity. Individual and clusters of charged amino acids were converted to alanines in an effort to generate a collection of conditionally defective Rep78/68 proteins. Rep78 variants were expressed in human 293 cells and analyzed for their ability to mediate replication of recombinant AAV vectors at various temperatures. The biochemical activities of Rep variants were further characterized in vitro by using Rep68 His-tagged proteins purified from bacteria. The results of these analyses identified a temperature-sensitive (ts) Rep protein (D40,42,44A-78) that exhibited a delayed replication phenotype at 32 degrees C, which exceeded wild-type activity by 48 h. Replication activity was reduced by more than threefold at 37 degrees C and was undetectable at 39 degrees C. Stability of the Rep78 protein paralleled replication levels at each temperature, further supporting a ts phenotype. Replication differences resulted in a 3-log-unit difference in virus yields between the permissive and nonpermissive temperatures (2.2 x 10(6) and 3 x 10(3), respectively), demonstrating that this is a relatively tight mutant. In addition to the ts Rep mutant, we identified a nonconditional mutant with a reduced ability to support viral replication in vivo. Additional characterization of this mutant demonstrated an Mg(2+)-dependent phenotype that was specific to Rep endonuclease activity and did not affect helicase activity. The two mutants described here are unique, in that Rep ts mutants have not previously been described and the D412A Rep mutant represents the first mutant in which the helicase and endonuclease functions can be distinguished biochemically. Further understanding of these mutants should facilitate our understanding of AAV replication and integration, as well as provide novel strategies for production of viral vectors.

Mutagenesis Reveals a Role for Epidermal Growth Factor Receptor Extracellular Subdomain IV in Ligand Binding

The extracellular domain of the epidermal growth factor (EGF) receptor (EGFR) comprises four subdomains (I-IV) and mediates binding of several different polypeptide ligands, including EGF, transforming growth factor-alpha, and heparin-binding EGF. Previous studies have predominantly implicated subdomain III in ligand binding. To investigate a possible role for sequences in subdomain IV, we constructed several mutant EGFRs in which clusters of charged or aromatic amino acids were replaced with alanine. Analysis of stably transfected Chinese hamster ovary cells expressing mutant EGFRs confirmed that they were present on the cell surface at levels approaching that of the wild-type receptor. Although tyrosine phosphorylation of most mutants was markedly induced by EGF, a cluster mutation (mt25) containing four alanine substitutions in the span of residues 521-527 failed to respond. EGF-induced tyrosine phosphorylation of an alternative mutant (DeltaEN) with amino acids 518-589 deleted was also greatly diminished. Larger doses of EGF or heparin-binding EGF induced only weak tyrosine phosphorylation of mt25, whereas the response to transforming growth factor-alpha was undetectable. These results suggest that mt25 might be defective with respect to either ligand binding or receptor dimerization. Quantitative analyses showed that binding of (125)I-EGF to mt25 and DeltaEN was reduced to near background levels, whereas binding of EGF to other cluster mutants was reduced 60-70% compared with wild-type levels. Among the mutants, only mt25 and DeltaEN failed to form homodimers or to transphosphorylate HER2/Neu in response to EGF treatment. Collectively, our results are the first to provide direct evidence that discrete subdomain IV residues are required for normal binding of EGF family ligands. Significantly, they were obtained with the full-length receptor in vivo, rather than a soluble truncated receptor, which has been frequently used for structure/function studies of the EGFR extracellular region.

Probing the activation of protein C by the thrombin-thrombomodulin complex using structural analysis, site-directed mutagenesis, and computer modeling.

Protein C (PC) is activated to an essential anticoagulant enzyme (activated PC or APC) by thrombin (T) bound to thrombomodulin (TM), a membrane receptor present on the surface of endothelial cells. The understanding of this complex biological system is in part limited due to the lack of integration of experimental and structural data. In the work presented here, we analyze the PC-T-TM pathway in the context of both types of information. First, structural analysis of the serine protease domain of PC suggests that a positively charged cluster of amino acids could be involved in the activation process. To investigate the importance of these basic amino acids, two recombinant PC mutants were constructed using computer-guided site-directed mutagenesis. The double mutant had the K62[217]N/K63[218]D substitution and in the single mutant, K86[241] was changed to S. Both mutants were activated by free thrombin at rates equivalent to that of wild-type PC (wt-PC) and they demonstrated similar calcium-dependent inhibition of their activation. The K86[241]S mutant and wt-PC were activated by thrombin bound to soluble TM at a similar rate. In contrast, the K62[217]N/ K63[218]D mutant was activated by the T-TM complex at a 10-fold lower catalytic efficiency due to a lowering in k(cat) and increase in Km. Molecular models for PC and thrombin bound to a segment of TM were developed. The experimental results and the modeling data both indicate that electrostatic interactions are of crucial importance to orient PC onto the T-TM complex. A key electropositive region centered around loops 37[191] and 60[214] of PC is defined. PC loop 37[191] is located 7-8 A from the TM epidermal growth factor (EGF) 4 while the loop 60[214] is about 10 A away from TM EGF4. Both loops are far from thrombin. A key function of TM could be to create an additional binding site for PC. The Gla domain of PC points toward the membrane and away from thrombin or the EGF modules of TM during the activation process.

Developmental expression and characterization of the alpha2,8-polysialyltransferase activity in embryonic chick brain.

The alpha2,8-polysialyltransferases (polySTs) from embryonic chick brain catalyze the alpha2,8-specific polysialylation of endogenous neural cell adhesion molecules (N-CAMs). This posttranslation glycosylation decreases N-CAM-dependent cell adhesion and migration. The enzymatic properties of the membrane-bound form of the polyST activity was investigated in vitro. Our results show that the polyST activity was developmentally expressed with maximum specific activity appearing about 12 days after fertilization. This time shortly precedes maximal expression of the cognate polysialylated N-CAMs. Kinetic studies showed the KMand Vmaxfor CMP-Neu5Ac were 133 microM and 0.13 microM/h, respectively, at pH 6.1, 33 degrees C. CMP-Neu5Gc was not a donor substrate. PolyST activity was increased 5- to 6-fold in the presence of 10 mM MnCl2,the preferred divalent cation, and 1 mM dithiothreitol (DTT). Heparin (3 kDa) was a noncompetitive inhibitor of polysialylation with a Kiof 9 microM. Based on the affinity of the enzyme for heparin, the polyST activity was partially purified ( approximately 30-fold) by heparin-Sepharose affinity chromatography, after differential solubilization with the zwitterionic detergent, CHAPS. DTT and chemical modification studies using the thiol-directed alkylating reagents, N-ethylmaleimide (NEM) and iodoacetamide (IAA), were used to show that at least one cysteinyl residue in the polyST was of critical importance for polysialylation, but of lesser importance for monosialylation, catalyzed by the alpha2,3-, alpha2,6-, and alpha2,8-monosialyltransferases (monoSTs). A sulfhydryl residue is implicated in chain initiation. Two important structural differences between the mono- and polySTs were revealed by sequence analyses. First, the polySTs contain heparin-like, positively charged amino acid clusters upstream of both sialylmotif L and S. Second, the polySTs contain a uniquely extended basic amino acid region (pI 11. 6-12.0) of 31 residues immediately upstream of sialylmotif S. This extended, positively charged region may function in the processive mechanism of polymerization by allowing nascent polySia chains to remain bound to the polyST during the repetitive addition of each new Sia residue to the nonreducing termini of the growing chain. The importance of these studies is that they provide new information on the enzymatic basis of polysialylation. They also reveal that sulfhydryl residues and extended basic amino acid domains are two structural features unique to polysialylation, in contrast to monosialylation. Both may be important distinguishing features between the classes of distributive (monoSTs) and processive polysialyltransferases, which have not been previously described.

Ezrin/radixin/moesin (ERM) proteins bind to a positively charged amino acid cluster in the juxta-membrane cytoplasmic domain of CD44, CD43, and ICAM-2.

CD44 has been identified as a membrane-binding partner for ezrin/radixin/moesin (ERM) proteins, plasma membrane/actin filament cross-linkers. ERM proteins, however, are not necessarily colocalized with CD44 in tissues, but with CD43 and ICAM-2 in some types of cells. We found that glutathione-S-transferase fusion proteins with the cytoplasmic domain of CD43 and ICAM-2, as well as CD44, bound to moesin in vitro. The regions responsible for the in vitro binding of CD43 and CD44 to moesin were narrowed down to their juxta-membrane 20-30-amino acid sequences in the cytoplasmic domain. These sequences and the cytoplasmic domain of ICAM-2 (28 amino acids) were all characterized by the positively charged amino acid clusters. When E-cadherin chimeric molecules bearing these positively charged amino acid clusters of CD44, CD43, or ICAM-2 were expressed in mouse L fibroblasts, they were co-concentrated with ERM proteins at microvilli, whereas those lacking these clusters were diffusely distributed on the cell surface. The specific binding of ERM proteins to the juxta-membrane positively charged amino acid clusters of CD44, CD43, and ICAM-2 was confirmed by immunoprecipitation and site-directed mutagenesis. From these findings, we conclude that ERM proteins bind to integral membrane proteins bearing a positively charged amino acid cluster in their juxta-membrane cytoplasmic domain.

Fibrinolytic properties of staphylokinase mutants obtained by ‘clustered charge-to-alanine’ mutagenesis

Summary The fibrinolytic properties of 18 mutants of recombinant staphylokinase (SakSTAR) in which clusters of two or three charged residues were converted to alanine (‘clustered charge-to-alanine scan’) were evaluated. The specific fibrinolytic activity, determined in a clot lysis time assay, of SakSTAR13 (with K11, D13 and D14 to A), SakSTAR48 (with E46 and K50 to A), and SakSTAR67 (with E65 and D69 to A) was Evaluation of the thrombolytic properties of selected SakSTAR moieties in hamsters with pulmonary embolism revealed dose-dependent clot lysis with wild-type SakSTAR (30–82% lysis with doses of 9–81 μg/kg), SakSTAR13 (32–63% lysis with doses of 81–750 μg/kg) and SakSTAR67 (27–73% lysis with doses of 81–250 μg/kg). 50% clot lysis was obtained at 7- to 10-fold higher dose for SakSTAR67 (175 μg/kg) or SakSTAR13 (250 μg/kg) than for wild-type SakSTAR (25 μg/kg). Thus, three clusters of charged amino acids were identified in SakSTAR (regions 11–14, 46–50 and 65–69) which are important for its fibrinolytic potency.

Interspecific RNA in situ hybridization reveals perinuclear mastermind transcripts in drosophila virilis

Interspecific and intraspecific comparisons of protein coding and noncoding nucleotide sequences have improved our understanding of the function and molecular evolution of numerous genes. In addition to sequence comparisons, for complex developmental genes an interspecific comparison of RNA expression patterns can be informative (Lukowitz et al. 1994). Preliminary data for such comparisons can be obtained from interspecific RNA in situ hybridization experiments. Using probes from Drosophila melanogaster, we examined the embryonic transcription pattern of the neurogenic gene mastermind (mam) in D. virilis. The analysis indicated that mam transcription is highly conserved in the two species and also revealed an unexpected feature of mam RNA local’ization during very early development. mam was identified through its role in the formation of the nervous system in the D. melanogaster embryo (Lehmann et al. 1983). Subsequent genetic analyses have demonstrated that mam has numerous roles in embryonic and postembryonic development (Ruohola et al. 199 1, Hartenstein et al. 1992). Molecular analyses reveal that mam encodes a nuclear protein and that the protein is highly repetitive, displaying numerous amino acid homopolymers. Nonrepetitive regions of mam show very limited similarity to sequences of known function, but prominent clusters of charged amino acids are evident. Studies of mam expression identified transcripts which accumulate in a spatially and temporally dynamic pattern (Smoller et al. 1990; Bettler, Schmid, and Yedvobnick 199 1; unpublished data). Interspecific nucleotide sequence comparison of the protein coding region of mam from D. melanogaster and D. virilis revealed an equilibrium between natural selection and molecular drive; extensive length variation in repetitive regions appears to be balanced by selection to maintain the overall amino acid length between two conserved clusters of

Expression and Characterization of Clustered Charge-to-Alanine Mutants of Low Mr Single-Chain Urokinase-Type Plasminogen Activator

In an effort to modify the fibrinolytic and/or pharmacokinetic properties of recombinant low M(r) single-chain urokinase-type plasminogen activator (rscu-PA-32k), mutants were prepared by site-directed mutagenesis of clusters of charged amino acids with the highest solvent accessibility. The following mutants of rscu-PA-32k were prepared: LUK-2 (Lys 212, Glu 213 and Asp 214 to Ala), LUK-3 (Lys 243 and Asp 244 to Ala), LUK-4 (Arg 262, Lys 264, Glu 265 and Arg 267 to Ala), LUK-5 (Lys 300, Glu 301 and Asp 305 to Ala) and LUK-6 (Arg 400, Lys 404, Glu 405 and Glu 406 to Ala). The rscu-PA-32k moieties were expressed in High Five Trichoplasiani cells, and purified to homogeneity from the conditioned cell culture medium, with recoveries of 0.8 to 3.7 mg/l. The specific fibrinolytic activities (220,000 to 300,000 IU/mg), the rates of plasminogen activation by the single-chain moieties and the rates of conversion to two-chain moieties by plasmin were comparable for mutant and wild-type rscu-PA-32k moieties, with the exception of LUK-5 which was virtually inactive. Equi-effective lysis (50% in 2 h) of 60 microliters 125I-fibrin labeled plasma clots submerged in 0.5 ml normal human plasma was obtained with 0.7 to 0.8 microgram/ml of wild-type or mutant rscu-PA-32k, except with LUK-5 (no significant lysis with 16 micrograms/ml). Following bolus injection in hamsters, all rscu-PA-32k moieties had a comparably rapid plasma clearance (1.3 to 2.7 ml/min), as a result of a short initial half-life (1.4 to 2.5 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Homopolymer length variation in the Drosophila gene mastermind

Runs of identical amino acids encoded by triplet repeats (homopolymers) are components of numerous proteins, yet their role is poorly understood. Large numbers of homopolymers are present in the Drosophila melanogaster mastermind (mam) protein surrounding several unique charged amino acid clusters. Comparison of mam sequences from D. virilis and D. melanogaster reveals a high level of amino acid conservation in the charged clusters. In contrast, significant divergence is found in repetitive regions resulting from numerous amino acid replacements and large insertions and deletions. It appears that repetitive regions are under less selective pressure than unique regions, consistent with the idea that homopolymers act as flexible spacers separating functional domains in proteins. Notwithstanding extensive length variation in intervening homopolymers, there is extreme conservation of the amino acid spacing of specific charge clusters. The results support a model where homopolymer length variability is constrained by natural selection.

The D domain of the thyroid hormone receptor alpha 1 specifies positive and negative transcriptional regulation functions.

Four structural domains are characteristic of the members of the thyroid/steroid receptor superfamily. Of these, the A/B and D domains are the least conserved. We have investigated the role of two clusters of positively charged amino acids within the D domain of the thyroid hormone receptor alpha 1 (TR alpha 1). The sequences Lys134-Arg-Lys and Arg188-Arg-Lys, individually or together, were substituted to the neutral residues TIT in three mutants named alpha 1-1, alpha 1-2, and alpha 1-3, respectively. Subcellular localization of transiently transfected wild-type and mutated TRs was monitored by immunostaining, using a TR alpha 1-specific antibody. The wild-type and the alpha 1-2 TRs were detected exclusively in the nucleus, in the presence or absence of thyroid hormone. In contrast, the alpha 1-1 and alpha 1-3 mutants accumulated in both cytoplasm and nucleus, underscoring the importance of the Lys134-Arg-Lys residues for correct nuclear targeting. More importantly, although the mutants had unimpaired DNA- and hormone-binding activities, all three had lost positive and negative transcriptional regulatory functions. Thus, transactivation and repression functions can be entirely dissociated from the other properties of the receptor. In addition, substitution of either one of the positively charged amino acid clusters was sufficient to convert the native TR alpha 1 into a dominant, thyroid hormone-dependent receptor antagonist. These observations, which underline the functional relevance of the D domain for TR alpha 1 function, may also have implications for the autosomal dominant syndrome of generalized resistance to thyroid hormone.

Scanning mutagenesis of interleukin-8 identifies a cluster of residues required for receptor binding.

In order to identify residues required for the binding of interleukin-8 (IL-8) to its receptor, mutants were constructed in which clusters of charged amino acids were systematically replaced with alanine along the entire IL-8 sequence. The mutants were tested for their ability to induce a receptor-mediated rise in cytosolic free Ca2+, a property of wild-type IL-8 which can readily be detected by flow cytometry using neutrophils loaded with the calcium probe Indo-1. Eleven of the 12 mutants caused neutrophil calcium mobilization at 5 nM; the exception being a triple alanine mutant at positions K3, E4, and R6, which was inactive at all concentrations tested (150 nM maximum). A second set of mutants was generated in which residues 1-15 were individually mutated to alanine. Mutants E4A, L5A, or R6A were all inactive in the Ca2+ assay at 5 nM and competed poorly with 125I-IL-8 for neutrophil receptor binding; I10A, E4A, L5A, and R6A had approximately 30-, 100-, 100-, and 1000-fold reduced affinity, as compared with control IL-8, respectively. The nuclear magnetic resonance structure of IL-8 indicates that, in solution, the side chains of E4, L5, R6, and I10 point away from the core of the protein and do not participate in any intramolecular hydrogen bonds or salt bridges (Clore, G. M., and Gronenborn, A. M. (1991) J. Mol. Biol. 217, 611-620).

Interspecific comparison of the unusually repetitiveDrosophila locusmastermind

The mastermind gene of Drosophila melanogaster encodes a novel, highly repetitive nuclear protein required for neural development. To identify functionally important regions we have initiated an interspecific comparison of the gene in Drosophila virilis. Mastermind transcription and genomic organization are similar in both species and sequence analysis reveals significant conservation in a major cluster of charged amino acids. In contrast, extensive variation is noted in homopolymer domains that immediately flank the acidic cluster. Distinct patterns of evolutionary change can be identified: the major difference between unique regions are occasional amino acid substitutions whereas the repetitive areas are characterized by numerous large in-frame insertions/deletions and a nearly threefold higher rate of amino acid replacement. Conservation of the acidic domain suggests that it has an important functional role whereas the hypervariable homopolymer regions appear to be under less selective constraints than adjacent unique areas.

Location of a threonine residue in the α -subunit M2 transmembrane segment that determines the ion flow through the acetylcholine receptor channel

By the combination of cDNA manipulation and functional analysis of normal and mutant acetylcholine receptor (AChR) channels of Torpedo expressed in Xenopus laevis oocytes determinants of ion flow were localized in the bends bordering the putative M2 transmembrane segment (Imoto et al. 1988). We now report that in the rat muscle AChR, substitution of a threonine residue in the alpha-subunit localized in the M2 transmembrane segment increases or decreases the channel conductance, depending on the size of the amino acid side chain located at this position. This threonine residue (alpha T264) is located adjacent to the cluster of charged amino acids that form the intermediate anionic ring (Imoto et al. 1988). This effect is pronounced for the large alkali cations Cs+, Rb+, K+ whereas for Na+ the effect is much smaller. Taken together the results suggest that the threonine residues at position 264 in the two alpha-subunits together with the amino acids of the intermediate anionic ring form part of a narrow region close to the cytoplasmic mouth of the AChR channel.

Nereis cuticle collagen: Relationship of fiber ultrastructure to biochemical and biophysical properties

The cuticles of annelid worms are composed of many layers of collagenous fibers. The collagens of annelid cuticles have chemical compositions that set them apart from other collagens, have molecular lengths of about 1500 to 1800 nm and are apparently not crosslinked. The distinctive staining pattern that arises from the regular quarter-staggered arrangement of molecules in Type I collagen fibers is not observed in annelid cuticle collagen fibers. To determine the reason for this complete absence of banding, we have examined the cuticle fibers of Nereis virens by X-ray diffraction, light microscopy, and electron microscopy. Novel techniques for making segment-long-spacing crystallites (SLS) from this collagen are described. Our results suggest that the packing arrangement of these unusual molecules is responsible for the lack of banding. The apparently random axial packing arrangement of cuticle collagen molecules results in an absence of regularly spaced gaps between the molecules and in nonaligned charged amino acid clusters. The strength of the cuticle fibers may be derived from the numerous hydrogen bonds and salt bridges that are potentially possible with such extremely long molecules.