Identification of novel interaction partners for the conserved membrane proximal region of alpha-integrin cytoplasmic domains

The human REV1 gene codes for a DNA template-dependent dCMP transferase

DNA is frequently damaged by various physical and chemical agents. DNA damage can lead to mutations during replication. In the yeast Saccharomyces cerevisiae, the damage-induced mutagenesis pathway requires the Rev1 protein. We have isolated a human cDNA homologous to the yeast REV1 gene. The human REV1 cDNA consists of 4255 bp and codes for a protein of 1251 amino acid residues with a calculated molecular weight of 138 248 Da. The human REV1 gene is localized between 2q11.1 and 2q11.2. We show that the human REV1 protein is a dCMP transferase that specifically inserts a dCMP residue opposite a DNA template G. In addition, the human REV1 transferase is able to efficiently and specifically insert a dCMP opposite a DNA template apurinic/apyrimidinic (AP) site or a uracil residue. These results suggest that the REV1 transferase may play a critical role during mutagenic translesion DNA synthesis bypassing a template AP site in human cells. Consistent with its role as a fundamental mutagenic protein, the REV1 gene is ubiquitously expressed in various human tissues.

Mutation enhancement by DINB1, a mammalian homologue of the Escherichia coli mutagenesis protein dinB

BACKGROUND

The Escherichia coli dinB gene is an SOS gene known to be required for lambda phage untargeted mutagenesis. When over-expressed, it exhibits a potent mutagenic activity without any exogenous treatment to damage DNA. Frameshift mutations at a run of identical bases are most enhanced. The product DinB is structurally related to the E. coli UmuC protein and the Saccharomyces cerevisiae Rev1 and Rad30 proteins, all of which are shown to be involved in bypass synthesis at a DNA lesion.

RESULTS

We have cloned and sequenced human and mouse cDNAs encoding a DinB homologue. Their products are highly similar to DinB and less similar to UmuC, Rev1 or Rad30, and hence the genes were named DINB1 for human and Dinb1 for mouse. Both genes were expressed most abundantly in testis. Transient expression of the mouse cDNA in cultured mouse cells resulted in a nearly 10-fold increase in the incidence of point mutations, among which about 30% were frameshift mutations.

CONCLUSIONS

The above results suggest that a mutagenic mechanism, a so-called untargeted type, also operates in mammalian cells. Taken together with recent findings that human cells have multiple DNA polymerases for translesion synthesis which are homologous to the S. cerevisiae Rev3 and Rad30 proteins, our results imply that multiple mutagenic pathways are conserved from bacteria to higher eukaryotes.

Replication of damaged DNA: molecular defect in xeroderma pigmentosum variant cells

Individuals with Xeroderma pigmentosum (XP) syndrome have a genetic predisposition to sunlight-induced skin cancer. Genetically different forms of XP have been identified by cell fusion. Cells of individuals expressing the classical form of XP (complementation groups A through G) are deficient in the nucleotide excision repair (NER) pathway. In contrast, the cells belonging to the variant class of XP (XPV) are NER-proficient and are only slightly more sensitive than normal cells to the killing action of UV light radiation. The XPV fibroblasts replicate damaged DNA generating abnormally short fragments either in vivo [A.R. Lehmann, The relationship between pyramidine dimers and replicating DNA in UV-irradiated human fibroblasts, Nucleic Acids Res. 7 (1979) 1901-1912; S.D. Park, J.E. Cleaver, Postreplication repair: question of its definition and possible alteration in Xeroderma pigmentosum cell strains, Proc. Natl. Acad. Sci. U.S.A. 76 (1979) 3927-3931.] or in vitro [S.M. Cordeiro, L.S. Zaritskaya, L.K. Price, W.K. Kaufmann, Replication fork bypass of a pyramidine dimer blocking leading strand DNA synthesis, J. Biol. Chem. 272 (1997) 13945-13954; D.L. Svoboda, L.P. Briley, J.M. Vos, Defective bypass replication of a leading strand cyclobutane thymine dimer in Xeroderma pigmentosum variant cell extracts, Cancer Res. 58 (1998) 2445-2448; I. Ensch-Simon, P.M. Burgers, J.S. Taylor, Bypass of a site-specific cis-syn thymine dimer in an SV40 vector during in vitro replication by HeLa and XPV cell-free extracts, Biochemistry 37 (1998) 8218-8226.], suggesting that in XPV cells, replication has an increased probability of being blocked at a lesion. Furthermore, extracts from XPV cells were found to be defective in translesion synthesis [A. Cordonnier, A.R. Lehmann, R.P.P. Fuchs, Impaired translesion synthesis in Xeroderma pigmentosum variant extracts, Mol. Cell. Biol. 19 (1999) 2206-2211.]. Recently, Masutani et al. [C. Masutani, M. Araki, A. Yamada, R. Kusomoto, T. Nogimori, T. Maekawa, S. Iwai, F. Hanaoka, Xeroderma pigmentosum variant (XP-V) correcting protein from HeLa cells has a thymine dimer bypass DNA polymerase activity, EMBO J. 18 (1999) 3491-3501.] have shown that the XPV defect can be corrected by a novel human DNA polymerase, homologue to the yeast DNA polymerase eta, which is able to replicate past cyclobutane pyrimidine dimers in DNA templates. This review focuses on our current understanding of translesion synthesis in mammalian cells whose defect, unexpectedly, is responsible for the hypermutability of XPV cells and for the XPV pathology.

Interactions of fibroblasts with the extracellular matrix: implications for the understanding of fibrosis

The cellular organization and the compartmentalization in multicellular organisms is mediated by the extracellular matrix (ECM). This structure is composed by a wide variety of different macromolecules which carry distinct domains with defined structural and/or biological activities. Cells are known to interact with these molecules via specific receptors. Following activation, these receptors transduce signals either directly to the intracellular cytoskeleton or via different signalling cascades. Cell-matrix interactions, therefore, not only control the shape and orientation of cells but can also directly regulate cellular functions, including migration, differentiation, proliferation, and the expression of different genes. These cell-matrix interactions have been elucidated in detail for several biological processes, especially morphogenesis and differentiation, but also play an important role during pathological situations, e.g. wound healing and tumor progression. Although much less investigated, similar mechanisms are thought to regulate the biological behavior of fibroblastic cells, the final target cells in fibrosis. The activity of these cells depends in various ways on the presence of ECM molecules. First, some of the molecules are known to bind to and modulate the activity of those growth factors and cytokines, which lead to the activation of fibroblasts during the early phases of fibrosis. Second, deposition of large amounts of ECM molecules alters the environment and the mechanical load on the cells which are embedded in this matrix. Third, ECM molecules directly modulate fibroblast metabolism via certain integrin receptors. This review summarizes recent developments in all three domains. It mainly focuses on the direct role of ECM molecules in the biosynthetic activity of fibroblasts.