Human Protein Complex Research Articles

Identification of two distinct human SMC protein complexes involved in mitotic chromosome dynamics.

The structural maintenance of chromosomes (SMC) family member proteins previously were shown to play a critical role in mitotic chromosome condensation and segregation in yeast and Xenopus. Other family members were demonstrated to be required for DNA repair in yeast and mammals. Although several different SMC proteins were identified in different organisms, little is known about the SMC proteins in humans. Here, we report the identification of four human SMC proteins that form two distinct heterodimeric complexes in the cell, the human chromosome-associated protein (hCAP)-C and hCAP-E protein complex (hCAP-C/hCAP-E), and the human SMC1 (hSMC1) and hSMC3 protein complex (hSMC1/hSMC3). The hCAP-C/hCAP-E complex is the human ortholog of the Xenopus chromosome-associated protein (XCAP)-C/XCAP-E complex required for mitotic chromosome condensation. We found that a second complex, hSMC1/hSMC3, is required for metaphase progression in mitotic cells. Punctate vs. diffuse distribution patterns of the hCAP-C/hCAP-E and hSMC1/hSMC3 complexes in the interphase nucleus indicate independent behaviors of the two complexes during the cell cycle. These results suggest that two distinct classes of SMC protein complexes are involved in different aspects of mitotic chromosome organization in human cells.

The mouse homolog of the bovine leukemia virus receptor is closely related to the delta subunit of adaptor-related protein complex AP-3, not associated with the cell surface.

A mouse cDNA (mBLVR1) which was highly homologous to the bovine cDNA of the bovine leukemia virus receptor (BLVR) gene was cloned. The mBLVR1 cDNA, of 4,730 bp, covered nearly the full length of the mRNA (about 5 kb) and included an open reading frame (ORF) encoding a protein of 1,199 amino acids. While the bovine BLVR protein was thought to be a type I transmembrane protein, the deduced protein coded by mBLVR1 did not appear to be a typical transmembrane protein. The ORF of mBLVR1 ended at a site 280 amino acids upstream of the termination codon of the bovine BLVR ORF, so the deduced mouse BLVR protein lacked the corresponding transmembrane and cytoplasmic regions of the predicted bovine BLVR protein. No significant hydrophobic region was found in the mouse protein. Recently, a human cDNA which was highly homologous (69.6% homology) to the mouse BLVR gene was reported. The cDNA encodes the delta subunit of the human adaptor-related protein complex AP-3, which aligned almost collinearly with the mouse BLVR protein. AP-3 and all other related adaptor protein complexes have been shown to be associated with intracellular vesicles but not with the cell surface. Thus, the mouse BLVR homolog appeared to be the mouse AP-3 delta subunit itself or closely related to it, but the bovine BLVR gene seemed slightly different from the adaptor subunit gene family.

RING1 is associated with the polycomb group protein complex and acts as a transcriptional repressor.

The Polycomb (Pc) protein is a component of a multimeric, chromatin-associated Polycomb group (PcG) protein complex, which is involved in stable repression of gene activity. The identities of components of the PcG protein complex are largely unknown. In a two-hybrid screen with a vertebrate Pc homolog as a target, we identify the human RING1 protein as interacting with Pc. RING1 is a protein that contains the RING finger motif, a specific zinc-binding domain, which is found in many regulatory proteins. So far, the function of the RING1 protein has remained enigmatic. Here, we show that RING1 coimmunoprecipitates with a human Pc homolog, the vertebrate PcG protein BMI1, and HPH1, a human homolog of the PcG protein Polyhomeotic (Ph). Also, RING1 colocalizes with these vertebrate PcG proteins in nuclear domains of SW480 human colorectal adenocarcinoma and Saos-2 human osteosarcoma cells. Finally, we show that RING1, like Pc, is able to repress gene activity when targeted to a reporter gene. Our findings indicate that RING1 is associated with the human PcG protein complex and that RING1, like PcG proteins, can act as a transcriptional repressor.

Interaction between the human nuclear cap-binding protein complex and hnRNP F.

hnRNP F was identified in a screen for proteins that interact with human CBP80 and CBP20, the components of the nuclear cap-binding complex (CBC). In vitro interaction studies showed that hnRNP F can bind to both CBP20 and CBP80 individually. hnRNP F and CBC bind independently to RNA, but hnRNP F binds preferentially to CBC-RNA complexes rather than to naked RNA. The hnRNP H protein, which is 78% identical to hnRNP F and also interacts with both CBP80 and CBP20 in vitro, does not discriminate between naked RNA and CBC-RNA complexes, showing that this effect is specific. Depletion of hnRNP F from HeLa cell nuclear extract decreases the efficiency of pre-mRNA splicing, a defect which can be partially compensated by addition of recombinant hnRNP F. Thus, hnRNP F is required for efficient pre-mRNA splicing in vitro and may participate in the effect of CBC on pre-mRNA splicing.

Reconstitution of Human Excision Nuclease with Recombinant XPF-ERCC1 Complex

The human XPF-ERCC1 protein complex is one of several factors known to be required for general nucleotide excision repair. Genetic data indicate that both proteins of this complex are necessary for the repair of interstrand cross-links, perhaps via recombination. To determine whether XPF-ERCC1 completes a set of six proteins that are sufficient to carry out excision repair, the human XPF and ERCC1 cDNAs were coexpressed in Sf21 insect cells from a baculovirus vector. The purified complex contained the anticipated 5' junction-specific endonuclease activity that is stimulated through a direct interaction between XPF and replication protein A (RPA). The recombinant complex also complemented extracts of XP-F cells and Chinese hamster ovary mutants assigned to complementation groups 1, 4, and 11. Furthermore, reconstitution of the human excision nuclease was observed with a mixture of five repair factors (XPA, XPC, XPG, TFIIH, and RPA) and the recombinant XPF-ERCC1, thus verifying that no additional protein factors are needed for the specific dual incisions characteristic of human excision repair.

Dissection of Functional Domains of the Human DNA Replication Protein Complex Replication Protein A

Replication protein A (RPA) is a mammalian single-stranded DNA binding factor essential for DNA replication, repair, and recombination. It is composed of three subunits of 70, 34, and 13 kDa (Rpa1, Rpa2, and Rpa3, respectively). Deletion mapping of the Rpa2 subunit identified the domain required for interaction with Rpa1 and Rpa3 which does not include the N-terminal domain that is phosphorylated during S phase. Deletion mapping of Rpa1 defined three domains. The C-terminal third of the Rpa1 polypeptide binds Rpa2 which itself forms a bridge between Rpa1 and Rpa3. The N-terminal third of Rpa1 bound single-stranded DNA under low stringency conditions only (0.1 M NaCl), while a central domain binds to single-stranded DNA under both low and high stringency conditions (0.5 M NaCl). Binding to p53 requires the N-terminal third of Rpa1 with some contribution from the C-terminal third. The evolutionarily conserved putative zinc finger near the C terminus of Rpa1 was not required for binding to single-stranded DNA, Rpa2, or p53. However, all three subdomains of Rpa1 and the zinc finger were essential for supporting DNA replication in vitro. These experiments are a first step toward defining peptide components responsible for the many functions of the RPA protein complex.

Replication of bovine papillomavirus type 1 origin-containing DNA in crude extracts and with purified proteins.

The in vitro replication of DNA containing the bovine papillomavirus (BPV-1) origin has been carried out with cell-free extracts from mouse FM3A and human HeLa cells. DNA synthesis required the E1 protein, the minimal origin of replication (nucleotides 7911-22 of the BPV-1 genome), and, at low levels of FM3A extract, the addition of the human single-stranded DNA-binding protein (also called RP-A or RF-A). The E2 protein was not absolutely required, but could stimulate DNA synthesis at low levels of E1. DNA synthesis was also reconstituted using purified proteins from HeLa cells. These protein factors included human single-stranded DNA-binding protein, topoisomerase I, and DNA polymerase (pol) alpha-primase complex. At low concentrations of pol alpha-primase complex, the formation of high molecular weight products was dependent on the addition of DNA polymerase delta holoenzyme containing proliferating cell nuclear antigen and activator 1, also called RF-C. We have overexpressed and isolated the E1 protein from bacteria. This protein also supported BPV DNA synthesis, both in crude extracts and with purified proteins suggesting that E1 phosphorylation is not required for BPV DNA replication in vitro.

Cloning of the large subunit of activator 1 (replication factor C) reveals homology with bacterial DNA ligases.

We have cloned a gene encoding a DNA-binding protein by Southwestern screening of a murine cDNA library with a double-stranded oligonucleotide containing the sequence from the bidirectional promoter of the alpha 1 and alpha 2 collagen IV genes. The middle portion of this 1131-amino acid protein has a region homologous to bacterial DNA ligases, and the more carboxyl portion contains several domains homologous to p40, p38, p37, and p36.5 subunits of activator 1 (A1, also called replication factor C), a human replication protein complex. Western blotting revealed that antiserum generated against part of the recombinant protein reacted specifically with the 145-kDa component of the purified human A1 complex, indicating that it is the murine counterpart of the A1 p145. Characterization of the DNA-binding activity of the recombinant fusion protein by gel mobility-shift assay revealed that it had a preference for a run of pyrimidines on one strand. Deletion analysis using recombinant proteins revealed that the DNA ligase-like domain was required for DNA-binding activity. The finding that the region required for the binding of murine A1 p145 to DNA has similarity to a domain found in DNA ligases suggests that this region may be utilized by both proteins in recognizing DNA.

Evidence that the transcription factor USF is a component of the human beta-globin locus control region heteromeric protein complex.

The human locus control region (LCR) consists of four DNase I hypersensitive sites upstream of the epsilon-globin gene and is intimately involved in globin gene transcription. We have used DNase I footprinting with K562 erythroleukemia cell extracts to identify protein components of the minimal LCR element, hypersensitive site 2. Six major regions of protection were observed, and the occupation of two regions (sites II and V) was strongly temperature-dependent. Fractionation of K562 nuclear proteins revealed a single major protein that bound tightly to site II. An E-box was necessary for high affinity binding to DNA. We used antibodies and recombinant USF protein to prove that the helix-loop-helix transcription factor USF is the only detectable component in K562 cells that binds to this site. Despite significant differences between site II and a canonical USF-binding site, the USF binding affinity was comparable for the two sites. In both cases the interaction with the E-box of either wild-type USF or a approximately 15-kDa minimal USF DNA binding polypeptide displays an unusual positive temperature dependence, consistent with the observed footprinting behavior. The results show that a relatively ubiquitous factor, not confined to erythroid cells, is an important part of the complex of proteins bound at hypersensitive site 2 of the LCR in K562 cells.

In vitro antimicrobial activity of the human neutrophil cytosolic S-100 protein complex, calprotectin, against Capnocytophaga sputigena.

Calprotectin is a complex of two anionic proteins found in abundance in the cytosol of neutrophils, certain macrophages, and oral epithelial keratinocytes. Bacteria of the genus Capnocytophaga are pathogens of periodontal origin which can cause systemic infection in neutropenic subjects. Recently, it has been observed that Capnocytophaga may be internalized by neutrophils within the cytosol rather than within a membrane-delimited phagosome. The purpose of this study was to test the in vitro antibacterial effect of the cytosolic complex, calprotectin, against Capnocytophaga sputigena. Calprotectin was purified from the cytosol of human neutrophils by gel filtration and anion exchange FPLC, and it exerted potent in vitro antimicrobial effects against C. sputigena. Net bacteriostatic activity was exerted up to 18 h, after which bactericidal effects were observed. Both net bacteriostatic and bactericidal activity occurred at concentrations above 20 micrograms/mL and exhibited identical dose-response characteristics. Particle counts increased in the presence of calprotectin, despite net bacteriostasis as assessed by changes in colony-forming units (CFU). Dose-response characteristics and direct particle counts suggested that net bacteriostatic effects were the result of balanced cell division and death, rather than suspension of cell division. We conclude that calprotectin can be a significant contributor to host defense against infection by Capnocytophaga.

The domain structure of the inhibitor subunit of human inter-α-trypsin inhibitor reflects the exon structure of its gene

The gene coding for the inhibitor subunit of the human plasma protein complex inter-α-trypsin inhibitor has been cloned. The exon structure of the gene corresponds with the organization of the protein in two distinct inhibitor domains. The exons coding for the inhibitor domains each comprise the complete information for a typical Kunitz-type proteinase inhibitor structure. In contrast to bovine aprotinin, the mature inhibitor protein contains amino acid residues flanking the inhibitor domains on both sides. These amino acid residues are encoded by additional exons.

Expression of human salivary protein genes.

Human proline-rich proteins (PRPs) are polymorphic, homologous in sequence, and linked in a cluster called the human salivary protein complex (SPC). Recently this complex was localized to human chromosome band 12p13.2 (Mamula et al., Cytogenet. Cell Genet. 39:279, 1985). We have isolated a PRP cDNA, EO27, from a human parotid gland library, identified it by DNA sequencing, and used it to study the molecular and cellular biology of PRP production. Cell-free translation and mRNA characterization with EO27 indicate that the numerous PRPs seen in saliva are produced from relatively few, large precursors, probably by posttranslational cleavage. This supports an hypothesis originally proposed by Friedman and Karn in 1977 (Am. J. Hum. Genet. 29:44 A; Biochem. Genet. 15:549) and later supported by biochemical studies (Karn et al., Biochem Genet. 17:1061, 1979) and molecular studies (Mamula et al., Fed. Proc. 43:1522, 1984; Maeda et al., J. Biol. Chem. 260:1123, 1985). EO27 was also used in this study to localize PRP mRNA production to the acinar cells of the parotid gland by in situ hybridization.

Localization of the human salivary protein complex (SPC) to chromosome band 12p13.2.

In situ hybridization of a 3H-labeled probe containing a fragment from PRP-1, a genomic clone with human salivary proline-rich protein gene sequences, revealed significant labeling on the short arm of human chromosome 12 in metaphase preparations from two individuals. Fifty-three percent of metaphases exhibited labeling on one or both chromosomes 12. Additional cells scored at the 850-1,000 band level revealed a significant proportion (52% [32/61] grains, p less than 0.005) of the labeled sites on chromosome 12 to be on band 12p13.2. This probe for a human salivary proline-rich protein gene fragment, probably PMS, is from a cluster of 13 linked genes designated as the human salivary protein complex (SPC). Studies of the DNA of human-mouse somatic-cell hybrids have assigned the SPC to chromosome 12, but have not provided a regional localization (Azen et al, 1985). This paper reports the localization of the SPC to a specific chromosomal band, 12p13.2.

Interaction of a human plasma lipid transfer protein complex with lipid monolayers

The interaction of a purified human plasma lipid transfer complex with cholesteryl ester, triacylglycerol and phosphatidylcholine in binary and ternary lipid monolayers was investigated. The lipid transfer complex, designated LTC, catalyzes the removal of cholesteryl oleate and triacylglycerol from phosphatidylcholine monolayers. Preincubation of LTC with p- chloromercuriphenyl sulfonate inhibits LTC-catalyzed removal of triacylglycerol; cholesteryl ester removal is not affected. The rate of LTC-facilitated removal of cholesteryl oleate from a phosphatidylcholine monolayer depends on the amount of LTC added to the subphase up to 100 μg protein. In addition, the rate of the LTC-catalyzed transfer of cholesteryl oleate to the subphase increases linearly as the amount of cholesteryl oleate in the monolayer increases to 6 mol%. LTC also removes cholesterol from phosphatidylcholine-cholesterol monolayers, albeit at a rate which is 15% of that for removal of cholesteryl oleate. The ability of LTC to facilitate triacylglycerol and cholesteryl ester removal depends on the composition of the monolayer. Phosphatidylcholine supports cholesteryl ester transfer whereas sphingomyelin-cholesteryl ester monolayers are almost refractory to LTC. In contrast, LTC removes triacylglycerol from either a phosphatidylcholine or a sphingomyelin monolayer. The results suggest the existence of at least two lipid transfer proteins, one of which catalyzes the removal of cholesteryl ester and the other triacylglycerol. The role of these proteins as they relate to lipoprotein metabolism is discussed.

Combination of Specific Antibodies with the Human Vitamin A-transporting Protein Complex

The interactions of specific antibodies with the human retinol-binding protein (RBP) and the thyroxine-binding prealbumin have been investigated. When an antibody combines with either of these proteins, the tryptophyl fluorescence of the antibody is partially quenched. Polarization of fluorescence of RBP-retinol increased rapidly on formation of antibody-RBP complexes. With fluorescence-quenching and polarization measurements as indicators, the stoichiometries and equilibrium constants of the antigen-antibody reactions have been studied. The data show that RBP, whether free or in complex with prealbumin, exhibits identical reactivity with anti-RBP Fab′-fragments. The conclusion is reached that the prealbumin binding site of RBP is not a major antigenic structure. The data indicate that the number of antigenic sites on RBP is limited and is the factor which controls the number of antibodies bound per RBP molecule. By quantum yield measurements of retinol it was found that RBP does not undergo any measureable conformational change on complex formation with Fab′-fragments. Free prealbumin could simultaneously interact with a maximum number of 12 anti-prealbumin Fab′-fragments. This suggests that there are three antigenic sites on each of the four identical prealbumin subunits. RBP, on forming a complex with prealbumin, competed with four anti-prealbumin Fab′-fragments. These Fab′-fragments showed a limited heterogeneity indicating that they may be directed toward a single antigenic site. It is proposed that the competition between RBP and the anti-prealbumin Fab′-fragments may be interpreted as a consequence of a negative cooperativity on RBP forming a complex with prealbumin. The equilibrium constant for Fab′-fragments forming a complex with prealbumin was lowered on thyroxine binding to prealbumin. This result supports the earlier suggestion that thyroxine binding to prealbumin is an example of a negative homotropic interaction.

Conformational studies of the human vitamin A-transporting protein complex.

Conformational studies of the human vitamin A-transporting protein complex.

Studies on the Fluorescence of the Human Vitamin A-transporting Plasma Protein Complex and Its Individual Components

Abstract The fluorescence properties of the human prealbumin-retinol-binding protein (RBP) complex and of its individual components are described. At neutral pH, RBP and the protein complex have two fluorescence bands: one at 335 nm and the other, associated with the retinol, at 470 nm. Prealbumin has only a fluorescence at 335 nm (quantum yield 0.11). The quantum yield of retinol increased by 50% and the emission spectrum was blue shifted on RBP forming a complex with prealbumin. RBP, at low ionic strength where the binding of RBP and prealbumin is abolished, exhibited a decrease in the quantum yield of retinol. With the protein fluorescence of RBP and the fluorescence of its cofactor, it was also possible to show the occurrence of transfer of excitation energy from the protein moiety to the retinol (efficiency of transfer: 60%). Studies on the accessibility of the aqueous medium (or protons) to the retinol site revealed that on complex formation between RBP and prealbumin most of the retinol-solvent interactions are abolished, suggesting that one of the functions of prealbumin is to stabilize the retinol-binding site of RBP. The estimated apparent association constant is 2 x 107 m-1 for the interaction of retinol-containing RBP and prealbumin. The binding constant of prealbumin and vitamin A-free RBP was also determined. A similar value was found and it is thus concluded that retinol has no major influence on the tertiary structure of RBP.

Studies on the Interaction Between Prealbumin, Retinol-binding Protein, and Vitamin A

The dissociation of the human vitamin A-transporting protein complex into free prealbumin and free retinol-binding protein (RBP) was studied by gel chromatography under varying conditions of pH and ionic strength. The interaction between the two proteins was diminished at a low ionic strength and complete dissociation seemed to occur at an ionic strength of 0.002. Variations in pH did not appear to alter seriously the affinity between prealbumin and RBP as the prealbumin-RBP complex was the predominant component in the pH range of 8 to 3. The release of vitamin A from free RBP and from prealbumin-RBP was studied at various values of pH and ionic strength. A facilitated release of retinol from RBP was found at conditions of low ionic strength, whereas variations in pH gave rise to only moderate changes in the release velocity of vitamin A. The binding of prealbumin to RBP seemed to strengthen the interaction between RBP and retinol. These observations suggested a conformational change of one or both of the two proteins at low ionic strength. Sedimentation velocity analyses performed at various values of ionic strength indicated that RBP was amenable to conformational changes at a low ionic strength. Variations in ionic strength did not seem to alter the sedimentation behavior of prealbumin. Immunochemical analyses showed that the prealbumin-RBP complex dissociated in the presence of antibodies directed against RBP, whereas antibodies directed against prealbumin did not seem to affect the interaction between prealbumin and RBP. These data suggest that RBP is more liable to conformational changes than is prealbumin.