Human papillomavirus type 16 E7 protein inhibits DNA binding by the retinoblastoma gene product.

The human papillomavirus E7 gene can transform murine fibroblasts and cooperate with other viral oncogenes in transforming primary cell cultures. One biochemical property associated with the E7 protein is binding to the retinoblastoma tumor suppressor gene product (pRB). Biochemical properties associated with pRB include binding to viral transforming proteins (E1A, large T, and E7), binding to cellular proteins (E2F and Myc), and binding to DNA. The mechanism by which E7 stimulates cell growth is uncertain. However, E7 binding to pRB inhibits binding of cellular proteins to pRB and appears to block the growth-suppressive activity of pRB. We have found that E7 also inhibits binding of pRB to DNA. A 60-kDa version of pRB (pRB60) produced in reticulocyte translation reactions or in bacteria bound quantitatively to DNA-cellulose. Recombinant E7 protein used at a 1:1 or 10:1 molar ratio with pRB60 blocked 50 or greater than 95% of pRB60 DNA-binding activity, respectively. A mutant E7 protein (E7-Ala-24) with reduced pRB60-binding activity exhibited a parallel reduction in its blocking of pRB60 binding to DNA. An E7(20-29) peptide that blocks binding of E7 protein to pRB60 restored the DNA-binding activity of pRB60 in the presence of E7. Peptide E7(2-32) did not block pRB60 binding to DNA, while peptide E7(20-57) and an E7 fragment containing residues 1 to 60 partially blocked DNA binding. E7 species containing residues 3 to 75 were fully effective at blocking pRB60 binding to DNA. These studies indicate that E7 protein specifically blocks pRB60 binding to DNA and suggest that the E7 region responsible for this property lies between residues 32 and 75. The functional significance of these observations is unclear. However, we have found that a point mutation in pRB60 that impairs DNA-binding activity also blocks the ability of pRB60 to inhibit cell growth. This correlation suggests that the DNA-binding activity of retinoblastoma proteins contributes to their biological properties.     

Three regions of the pRB pocket domain affect its inactivation by human papillomavirus E7 proteins

A critical event in papillomavirus transformation of human cells is the inactivation of pRB by the E7 protein. E7, like many other viral oncoproteins, possesses a well-characterized LXCXE peptide motif that interacts with the pocket domain of pRB. Disruption of the LXCXE-binding cleft on pRB renders it resistant to E7 binding and inactivation. Such binding cleft mutants of pRB are capable of inducing a G(1) arrest in the human papillomavirus 18-transformed HeLa cell line. We show here that the efficient inactivation of pRB in HeLa cells does not simply depend on the integrity of the LXCXE-binding cleft. Multiple site-directed mutants that alter conserved surfaces of the pRB pocket domain cause HeLa cells to accumulate in G(1). We divide these mutants into two classes: those that can be bound by E7 and those that cannot. The E7 interacting mutants include changes in conserved residues that lie in a groove between the A and B halves of the pocket. Surprisingly, none of these mutants show a clear defect in any of the known mechanisms for pRB inactivation by E7. Analysis of mutants that are compromised for E7 binding reveals that this interaction depends on both the LXCXE-binding cleft and on a conserved group of lysines adjacent to the cleft. These basic amino acids on pRB define a discrete interaction point with E7. These residues most likely form ionic interactions with conserved acidic amino acids on E7 since a stable pRB/E7 interaction was restored when the lysine residues on pRB and the acidic residues on E7 were interchanged.     

Identification of HPV-16 E7 peptides that are potent antagonists of E7 binding to the retinoblastoma suppressor protein

Complex formation between the human papilloma virus type-16 E7 protein (HPV-16 E7) and the retinoblastoma suppressor protein (pRB) is believed to be important in the process of cellular transformation that leads to cervical carcinoma. Utilizing an in vitro solution assay as well as a plate binding assay that measures the association between HPV-16 E7 and pRB proteins, we have examined a series of synthetic HPV-16 E7 peptides. HPV-16 E7 peptides which lie between amino acid residues 14 and 32 were found to be potent inhibitors of E7/pRB binding. The minimal peptide structure that possessed full antagonist activity was N-acetyl-E7-(21-29)-peptide amide. This peptide inhibited 100% of E7/pRB binding and exhibited an IC50 of 40 nM in the plate binding assay. A purified beta-galactosidase-E7 fusion protein exhibited an IC50 of 2 nM in the same assay. These results suggest that other regions of the E7 molecule in addition to amino acids 21-29 may contributed to E7/pRB interaction. Analysis of E7-(20-29)-peptides containing single amino acid substitutions suggests that Cys24, Tyr23, Tyr25, Asp21, and Glu26 are important residues for maintaining maximal antagonist activity. This series of peptides should prove useful in analyzing the biological consequences of E7/pRB binding in HPV-infected cells.     

Effects of active site mutations in haemoglobin I from Lucina pectinata: a molecular dynamic study

Haemoglobin I from Lucina pectinata is a monomeric protein consisting of 142 amino acids. Its active site contains a peculiar arrangement of phenylalanine residues (PheB10, PheCD1 and PheE11) and a distal Gln at position E7. Active site mutations at positions B10, E7 and E11 were performed in deoxy haemoglobin I (HbI), followed by 10 ns molecular dynamic simulations. The results showed that the mutations induced changes in domains far from the active site producing more flexible structures than the native HbI. Distance analyses revealed that the heme pocket amino acids at positions E7 and B10 are extremely sensitive to any heme pocket residue mutation. The high flexibility observed by the E7 position suggests an important role in the ligand binding kinetics in ferrous HbI, while both positions play a major role in the ligand stabilisation processes. Furthermore, our results showed that E11Phe plays a pivotal role in protein stability.     

The Arabidopsis TAG1 transposase has an N-terminal zinc finger DNA binding domain that recognizes distinct subterminal motifs

The in vitro DNA binding activity of the Arabidopsis Tag1 transposase (TAG1) was characterized to determine the mechanism of DNA recognition. In addition to terminal inverted repeats, the Tag1 element contains four different subterminal repeats that flank a transcribed region encoding a 729-amino acid protein. A single site-specific DNA binding domain is located near the N terminus of TAG1, between residues 21 and 133. This domain binds specifically to the AAACCC and TGACCC subterminal repeats, found near the 5' and 3' ends of the element, respectively. The ACCC sequence within these repeats is critical for recognition because mutations at positions 3, 5, and 6 abolished binding, yet the first two bases also are important because substitutions at these positions decreased binding by up to 90%. Weak interaction also occurs with the terminal inverted repeats, but no binding was observed to the other two 3' subterminal repeat regions. Sequence analysis of the TAG1 DNA binding domain revealed a C(2)HC zinc finger motif. Tests for metal dependence showed that DNA binding activity was inhibited by divalent metal chelators and greatly enhanced by zinc. Furthermore, mutation of each cysteine residue predicted to be a metal ligand in the C(2)HC motif abolished DNA binding. Together, these data show that the DNA binding domain of TAG1 specifically binds to distinct subterminal repeats and contains a zinc finger.     

Protein kinase C inhibits binding of diacylglycerol kinase-zeta to the retinoblastoma protein

We previously showed that the retinoblastoma protein (pRB), a key regulator of G1 to S-phase transition of the cell cycle, binds to and stimulates diacylglycerol kinase-zeta (DGKzeta) to phosphorylate the lipid second messenger diacylglycerol into phosphatidic acid. pRB binds to the MARCKS phosphorylation-site domain of DGKzeta that can be phosphorylated by protein kinase C (PKC). Here, we report that activation of PKC by phorbol ester inhibits DGKzeta binding to pRB. Ro 31-8220, a specific inhibitor of PKC, alleviated this inhibition of binding. Mimicking of PKC phosphorylation of serine residues (by S/D but not S/N mutations) within the DGKzeta-MARCKS phosphorylation-site domain also prevented DGKzeta binding to pRB, suggesting that PKC phosphorylation of these residues negatively regulates the interaction between DGKzeta and pRB. In PKC overexpression studies, it appeared that activation of particularly the (wild-type) PKCalpha isoform inhibits DGKzeta binding to pRB, whereas dominant-negative PKCalpha neutralized this inhibition. PKCalpha activation thus prevents DGKzeta regulation by pRB, which may have implications for nuclear diacylglycerol and phosphatidic acid levels during the cell cycle.     

The B-domain lysine patch of pRB is required for binding to large T antigen and release of E2F by phosphorylation

Cell cycle-dependent, site-specific phosphorylation of the retinoblastoma protein, pRB, is mediated by cyclin-dependent kinases (CDKs) and regulates the binding of pRB to many proteins. We previously showed that the interaction of pRB with E2F on DNA was regulated by the accumulation of phosphate groups on pRB. Here we show that positively charged lysine residues in the B domain of pRB are necessary for the release of pRB from E2F on DNA following phosphorylation by cyclin E-cdk2 kinase. These lysine residues are also important in the binding of the simian virus 40 large T antigen (TAg) to pRB, and mutation of these lysines to arginines alters the dependency of the pRB-TAg interaction on phosphorylation of pRB.     

Nonconserved lysine residues attenuate the biological function of the low-risk human papillomavirus E7 protein

The mucosotrophic human papillomaviruses (HPVs) are classified as high-risk (HR) or low-risk (LR) genotypes based on their neoplastic properties. We have demonstrated previously that the E7 protein destabilizes p130, a pRb-related pocket protein, thereby promoting S-phase reentry in postmitotic, differentiated keratinocytes of squamous epithelia, and that HR HPV E7 does so more efficiently than LR HPV E7. The E7 proteins of LR HPV-11 and -6b uniquely possess lysine residues following a casein kinase II phosphorylation motif which is critical for the biological function of E7. We now show that mutations of these lysine residues elevated the efficiency of S-phase reentry, independent of their charge. An 11E7 K39,42R mutation moderately increased the association with and the destabilization of p130. Unexpectedly, polyubiquitination on these lysine residues did not attenuate E7 activity, as their mutation caused elevated proteasomal degradation and decreased protein stability. In this regard, the biologically more potent HR HPV E7 proteins were also less stable than the LR HPV E7 proteins. We infer that these lysine residues impede functional protein-protein interactions. A G22D mutation of 11E7 at the pocket protein binding motif possessed augmented efficiency in promoting S-phase reentry and strongly enhanced association with p130 and pRb. The combined effects of these two classes of 11E7 mutations exhibited an efficiency of S-phase reentry comparable to that of HR HPV E7. Thus, these nonconserved residues are primarily responsible for the differential abilities of LR and HR HPV E7 proteins to promote unscheduled DNA replication in organotypic raft cultures.     

Polyomavirus large T mutants affected in retinoblastoma protein binding are defective in immortalization.

To clarify the relationship between the various activities of the polyomavirus large T antigen and the contribution of this oncogene to neoplastic transformation, we constructed a series of mutants with small deletions or single-amino-acid substitutions in two separate regions of the protein. These sequences were targeted because they showed considerable similarity to conserved regions 1 and 2 of adenovirus E1A which are thought to be binding sites for the retinoblastoma gene product (pRB). The pRB-binding properties of the large T mutants were assessed with an in vitro coimmunoprecipitation assay. pRB binding was readily detected with wild-type large T, but coprecipitation was completely abolished by as little as a single amino acid substitution (Asp-141----Glu or Glu-146----Asp) in region 2 of the polyomavirus large T antigen. Mutants defective in pRB binding were unable to immortalize primary rat embryo fibroblasts, suggesting that association with pRB is an important component of immortalization mediated by polyomavirus large T. The mutations in region 1 affected pRB binding only marginally, yet some of them severely impaired immortalization, indicating that pRB binding may be essential but not sufficient for immortalization.     

The zinc finger motif of Escherichia coli RecQ is implicated in both DNA binding and protein folding

The RecQ family of DNA helicases has been shown to be important for the maintenance of genomic integrity. Mutations in human RecQ genes lead to genomic instability and cancer. Several RecQ family of helicases contain a putative zinc finger motif of the C4 type at the C terminus that has been identified in the crystalline structure of RecQ helicase from Escherichia coli. To better understand the role of this motif in helicase from E. coli, we constructed a series of single mutations altering the conserved cysteines as well as other highly conserved residues. All of the resulting mutant proteins exhibited a high level of susceptibility to degradation, making functional analysis impossible. In contrast, a double mutant protein in which both cysteine residues Cys397 and Cys400 in the zinc finger motif were replaced by asparagine residues was purified to homogeneity. Slight local conformational changes were detected, but the rest of the mutant protein has a well defined tertiary structure. Furthermore, the mutant enzyme displayed ATP binding affinity similar to the wild-type enzyme but was severely impaired in DNA binding and in subsequent ATPase and helicase activities. These results revealed that the zinc finger binding motif is involved in maintaining the integrity of the whole protein as well as DNA binding. We also showed that the zinc atom is not essential to enzymatic activity.     

Mapping the rho1 GABA(C) receptor agonist binding pocket. Constructing a complete model

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian brain. The GABA receptor type C (GABA(C)) is a ligand-gated ion channel with pharmacological properties distinct from the GABA(A) receptor. To date, only three binding domains in the recombinant rho1 GABA(C) receptor have been recognized among six potential regions. In this report, using the substituted cysteine accessibility method, we scanned three potential regions previously unexplored in the rho1 GABA(C) receptor, corresponding to the binding loops A, E, and F in the structural model for ligand-gated ion channels. The cysteine accessibility scanning and agonist/antagonist protection tests have resulted in the identification of residues in loops A and E, but not F, involved in forming the GABA(C) receptor agonist binding pocket. Three of these newly identified residues are in a novel region corresponding to the extended stretch of loop E. In addition, the cysteine accessibility pattern suggests that part of loop A and part of loop E have a beta-strand structure, whereas loop F is a random coil. Finally, when all of the identified ligand binding residues are mapped onto a three-dimensional homology model of the amino-terminal domain of the rho1 GABA(C) receptor, they are facing toward the putative binding pocket. Combined with previous findings, a complete model of the GABA(C) receptor binding pocket was proposed and discussed in comparison with the GABA(A) receptor binding pocket.     

Functional analysis at the Cys706 residue of the retinoblastoma protein.

A missense mutation at cysteine 706, resulting in a retinoblastoma (RB) protein defective in phosphorylation and oncoprotein binding, has been isolated from a human tumor cell line. Since this residue is conserved in murine RB and in the related p107 protein, we studied the activity of in vitro mutants flanking this position. These experiments demonstrated that the thiol atom at codon 706 does not possess intrinsic functional activity as small polar or nonpolar residues could substitute at either codons 706 or 707, while bulkier R-group changes in these positions interfered with in vitro oncoprotein binding or in vivo protein phosphorylation. A series of missense mutants in an adjacent leucine repeat domain also demonstrated a loss of oncoprotein binding that was proportional to the magnitude of amino acid substitutions. To determine whether the cysteine 706 --> phenylalanine RB mutant retained any protein binding activity, we examined its ability to precipitate MYC, which was recently identified as a potential RB-associated protein. These experiments demonstrated that the mutant RB product is capable of binding in vitro to c-myc and L-myc proteins with comparable affinity as wild-type RB. These findings raise questions about the functional role of the RB:MYC interactions and emphasize important differences in the binding patterns between MYC and the other RB-associated proteins.     

苦荞谷胱甘肽转移酶(FtGST)基因的克隆与序列分析

采用RACE技术,从苦荞(Fagopyrum tatarium)中克隆得到一个谷胱甘肽转移酶(Glutathione S-transferase protein,FtGST)基因。序列分析表明,FtGST基因全长DNA序列和cDNA序列编码区分别为746 bp和666 bp,DNA序列含有一个长度为80 bp(342-421 bp)的内含子;开放阅读框(ORF)长666 bp,编码221个氨基酸。生物信息学分析表明,FtGST基因推导的蛋白质含有Tau家族典型的底物结合口袋、谷胱甘肽结合位点(G-site)和疏水性底物结合位点(H-site)氨基酸残基,表明FtGST为Tau家族蛋白。     

Zinc binding by the methylation signaling domain of the Escherichia coli Ada protein.

The Escherichia coli Ada protein repairs O6-methylguanine residues and methyl phosphotriesters in DNA by direct transfer of the methyl group to a cysteine residue located in its C- or N-terminal domain, respectively. Methyl transfer to the N-terminal domain causes it to acquire a sequence-specific DNA binding activity, which directs binding to the regulatory region of several methylation-resistance genes. In this paper we show that the N-terminal domain of Ada contains a high-affinity binding site for a single zinc atom, whereas the C-terminal domain is free of zinc. The metal-binding domain is apparently located within the first 92 amino acids of Ada, which contains four conserved cysteine residues. We propose that these four cysteines serve as the zinc ligand residues, coordinating the metal in a tetrahedral arrangement. One of the putative ligand residues, namely, Cys69, also serves as the acceptor site for a phosphotriester-derived methyl group. This raises the possibility that methylation-dependent ligand reorganization about the metal plays a role in the conformational switching mechanism that converts Ada from a non-sequence-specific to a sequence-specific DNA-binding protein.