Role of the LXCXE Binding Site in Rb Function

Abstract

Oncoproteins from DNA tumor viruses such as adenovirus E1a, simian virus 40 T antigen, and human papillomavirus E7 contain an LXCXE sequence, which they use to bind the retinoblastoma protein (Rb) and inhibit its function. Cellular proteins such as histone deacetylases 1 and 2 (HDAC1 and -2) also contain an LXCXE-like sequence, which they use to interact with Rb. The LXCXE binding site in Rb was mutated to assess its role in Rb function. These mutations inhibited binding to HDAC1 and -2, which each contain an LXCXElike sequence, but had no effect on binding to HDAC3, which lacks an LXCXE-like sequence. Mutation of the LXCXE binding site inhibited active transcriptional repression by Rb and prevented it from effectively repressing the cyclin E and A gene promoters. In contrast, mutations in the LXCXE binding site did not prevent Rb from binding and inactivating E2F. Thus, the LXCXE mutations appear to separate Rb's ability to bind and inactivate E2F from its ability to efficiently recruit HDAC1 and -2 and actively repress transcription.

The SWI/SNF complex and cancer

“…Other regions within BRG1 and BRM are also believed to facilitate RB family binding (Bourachot et al, 1999;Dahiya et al, 2000). The ATPase domains of BRM and BRG1 are composed of helicase and DEAD box domains, which function as the motor units that convert ATP energy to mechanical movement (Muchardt and Yaniv, 1999a).…”

“…The growth inhibitory effects of BRG1 are attenuated by the adenovirus E1A protein, which blocks the cell cycle checkpoint functions of RB family members RB1, p107 and p130. The observation that E1A blocks BRG1-mediated growth inhibition strongly suggests that SWI/ SNF activity is required for normal growth regulation by RB family members (Dunaief et al, 1994;Strober et al, 1996), and BRG1 and BRM contain the RB-binding motif LxCxE and bind RB as well as RB family members p107 and p130 (Dunaief et al, 1994;Strober et al, 1996;Dahiya et al, 2000). Constitutively active RB does not induce G1 arrest in cells that lack BRG1 and BRM expression (Strobeck et al, 2000(Strobeck et al, , 2001Zhang et al, 2000;Reisman et al, 2002).…”

Remodeling of chromatin structure in senescent cells and its potential impact on tumor suppression and aging

“…Other more distantly related enzymes include human hBrm, Brg1 and the CHD family (CHD1-7). Of note, human hBrm and Brg1 are two candidate tumor suppressor genes that are implicated in repression of E2F target genes and induction of cell differentiation and senescence, through physical and functional interactions with the pRB protein (33,38,56,76,85,104,105,115,124,134,142). Since E2F target genes are incorporated into SAHF and formation of SAHF requires an intact pRB pathway (88,140), hBrm and/or Brg1 might be involved in SAHF assembly, perhaps through deposition of macroH2A.…”

Retinoblastoma family proteins as key targets of the small DNA virus oncoproteins

“…All these three large T-antigens possess a J domain, the homologous of a region conserved among all cellular DnaJ/Hsp40 molecular chaperones, essential for chaperone activity and for the interaction with the DnaK homolog Hsp70/Hsc70 chaperone machinery Kelley and Georgopoulos, 1997;Srinivasan et al, 1997). Simian virus 40, JCV and BKV large T-antigen (LT-Ag) also possess the LxCxE motif, through which they physically interact with all the members of the RB family proteins (Dahiya et al, 2000), which in turn bind to the viral factors through their 'pocket' domain (Paggi et al, 1996;Helt and Galloway, 2003). Binding of LT-Ag, as well as E1A or E7, to the pocket region of the RB proteins results in the physical displacement of physiologically interacting partners, apart from the presence or the absence of a LxCxE motif (e.g.…”