The retinoblastoma protein {pll0 nB) interacts with many cellular proteins in complexes potentially important for its growth-suppressing [unction. We have developed and used an improved version of the yeast two-hybrid system to isolate human cDNAs encoding proteins able to bind pll0 RB. One clone encodes a novel type 1 protein phosphatase catalytic subunit (PP-la2), which differs from the originally defined PP-lc~ by an amino-terminal l 1-amino-acid insert. In vitro-binding assays demonstrated that PP-lc~ isoforms preferentially bind the hypophosphorylated form of p ll0 RB. Moreover, similar pll0 RB sequences are required for binding PP-lc~2 and SV40 large T antigen. Cell cycle synchrony experiments revealed that this association occurs from mitosis to early Gv The implications of these findings on the regulation of both proteins are discussed.

Mutations in the BRCA2 (breast cancer susceptibility gene 2) tumor suppressor lead to chromosomal instability due to defects in the repair of double-strand DNA breaks (DSBs) by homologous recombination, but BRCA2's role in this process has been unclear. Here, we present the 3.1 angstrom crystal structure of a approximately 90-kilodalton BRCA2 domain bound to DSS1, which reveals three oligonucleotide-binding (OB) folds and a helix-turn-helix (HTH) motif. We also (i) demonstrate that this BRCA2 domain binds single-stranded DNA, (ii) present its 3.5 angstrom structure bound to oligo(dT)9, (iii) provide data that implicate the HTH motif in dsDNA binding, and (iv) show that BRCA2 stimulates RAD51-mediated recombination in vitro. These findings establish that BRCA2 functions directly in homologous recombination and provide a structural and biochemical basis for understanding the loss of recombination-mediated DSB repair in BRCA2-associated cancers.

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BRCA1 encodes a tumor suppressor that is mutated in familial breast and ovarian cancers. Here, it is shown that BRCA1 interacts in vitro and in vivo with hRad50, which forms a complex with hMre11 and p95/nibrin. Upon irradiation, BRCA1 was detected in discrete foci in the nucleus, which colocalize with hRad50. Formation of irradiation-induced foci positive for BRCA1, hRad50, hMre11, or p95 was dramatically reduced in HCC/1937 breast cancer cells carrying a homozygous mutation in BRCA1 but was restored by transfection of wild-type BRCA1. Ectopic expression of wild-type, but not mutated, BRCA1 in these cells rendered them less sensitive to the DNA damage agent, methyl methanesulfonate. These data suggest that BRCA1 is important for the cellular responses to DNA damage that are mediated by the hRad50-hMre11-p95 complex.

To define a mechanism by which retinoblastoma protein (Rb) functions in cellular differentiation, we studied primary fibroblasts from the lung buds of wild-type (RB + / +) and null-mutant (RB-/-) mouse embryos. In culture, the RB +/+ fibroblasts differentiated into fat-storing cells, either spontaneously or in response to hormonal induction; otherwise syngenic RB -/-fibroblasts cultured in identical conditions did not. Ectopic expression of normal Rb, but not Rb with a single point mutation, enabled RB-/-fibroblasts to differentiate into adipocytes.

Mutational inactivation of the retinoblastoma susceptibility (RB) gene has been proposed as a crucial step in the formation of retinoblastoma and other types of human cancer. This hypothesis was tested by introducing, via retroviral-mediated gene transfer, a cloned RB gene into retinoblastoma or osteosarcoma cells that had inactivated endogenous RB genes. Expression of the exogenous RB gene affected cell morphology, growth rate, soft agar colony formation, and tumorigenicity in nude mice. This demonstration of suppression of the neoplastic phenotype by a single gene provides direct evidence for an essential role of the RB gene in tumorigenesis.

The retinoblastoma protein interacts with a number of cellular proteins to form complexes which are probably crucial for its normal physiological function. To identify these proteins, we isolated nine distinct clones by direct screening of cDNA expression libraries using purified RB protein as a probe. One of these clones, Apl2, is expressed predominantly at the G1-S boundary and in the S phase of the cell cycle. The nucleotide sequence of Apl2 has features characteristic of transcription factors. The C-terminal region binds to unphosphorylated RB in regions similar to those to which T antigen binds and contains a transactivation domain. A region containing a potential leucine zipper flanked by basic residues is able to bind an E2F recognition sequence specifically. Expression ofApl2 in mammalian cells significantly enhances E2F-dependent transcriptional activity. These results suggest that Apl2 encodes a protein with properties known to be characteristic of transcription factor E2F.The retinoblastoma gene (RB), the first tumor suppressor gene identified, encodes a nuclear phosphoprotein which is ubiquitously expressed in vertebrates (19,20,42,44 (45,47), fluctuates with the cell cycle (7, 11, 13); (ii) the unphosphorylated form of RB is present predominantly in the Go-G1 stage (11, 13); (iii) microinjection of unphosphorylated RB into cells at early G1 inhibits their progression into the S phase (22). These data suggest that RB serves as a critical regulator of entry into the cell cycle and that its inactivation in normal cells leads to deregulated growth.How RB functions is the subject of intense inquiry. Two known biochemical properties of the RB protein have been described; one is its intrinsic DNA-binding activity, which was mapped to its C-terminal 300 amino acid residues (44, 66); another is its ability to interact with several oncoproteins of DNA tumor viruses (12,16,67). This interaction was mapped to two discontinuous regions at amino acids 394 to 571 and 649 to 773, designated as the T-binding domains (29,33). Interestingly, mutations of RB protein in tumors were *

The BRCA2 gene was identified based on its involvement in familial breast cancer. The analysis of its sequence predicts that the gene encodes a protein with 3,418 amino acids but provides very few clues pointing to its biological function. In an attempt to address this question, specific antibodies were prepared that identified the gene product of BRCA2 as a 390-kDa nuclear protein. BRCA2 was identified (1, 2) based on its initial mapping to chromosome 13q12-13 by linkage analysis of families with inherited breast cancer not attributed to mutations in BRCA1 (3). Germ-line mutations in BRCA2 account for the same percentage of familial breast cancers as BRCA1 (1). Together, these two breast cancer susceptibility genes are responsible for a large percentage of familial cases. In addition to breast cancer, BRCA2 mutations are also linked to other cancers including ovarian (4, 5), hepatocellular (6), pancreatic (5, 7), and prostate (4-6) tumors. However, mutations in BRCA2, like BRCA1, are mainly found in familial breast cancer but seldom occur in sporadic cases (8, 9). There have been over 100 distinct mutations spanning the sequence of this large gene (Breast Cancer Information Core). The majority of these mutations lead to truncation of the gene product.BRCA2 has 27 exons and expresses an mRNA 11 kb in size (1). The expression pattern of BRCA2 mRNA is similar to that of BRCA1, with highest levels in the testis, thymus, and ovaries (10). During mouse development, Brca2 mRNA is first detected on embryonic day 7.5, a time of rapid proliferation (11). At the cellular level, expression is regulated in a cell cycledependent manner with peak expression of BRCA2 mRNA in S phase (12). These results suggest BRCA2 may have a role in proliferating cells. Using a gene knockout method to create mice with BRCA2 mutations, homozygous mutant mice with BRCA2 truncated from the 5Ј half of exon 11 cannot survive embryogenesis (refs. 11, 13, and 14, and our unpublished results), suggesting that Brca2, like Brca1, plays an essential role in early embryonic development. Similar to Brca1, Brca2 heterozygotes are phenotypically normal and fertile. Although they are predicted to be genetically predisposed to cancer, they show no evidence to date of increased tumor formation.The identification of the BRCA2 gene was accomplished, quickly giving rise to the hope that the function(s) of the gene product would soon become clear. However, BRCA2 presents dilemmas similar to BRCA1, as very little insight in its function has been determined. To address this question systematically, we have identified the cellular BRCA2 protein as a nuclear protein and determined the domain responsible for interactions with human RAD51. Furthermore, BRCA2 apparently has a critical role in response to DNA damage. These results provide a molecular basis that begins to explain how mutations of BRCA2 contribute to carcinogenesis.