Roles for the coactivators CBP and p300 and the APC/C E3 ubiquitin ligase in E1A-dependent cell transformation

Abstract

Adenovirus early region 1A (E1A) possesses potent transforming activity when expressed in concert with activated ras or E1B genes in in vitro tissue culture systems such as embryonic human retinal neuroepithelial cells or embryonic rodent epithelial and fibroblast cells. Early region 1A has thus been used extensively and very effectively as a tool to determine the molecular mechanisms that underlie the basis of cellular transformation. In this regard, roles for the E1A-binding proteins pRb, p107, p130, cyclic AMP response element-binding protein (CBP)/p300, p400, TRRAP and CtBP in cellular transformation have been established. However, the mechanisms by which E1A promotes transformation through interaction with these partner proteins are not fully delineated. In this review, we focus on recent advances in our understanding of CBP/p300 function, particularly with regard to its relationship to the anaphase-promoting complex/cyclosome E3 ubiquitin ligase, which has recently been shown to interact and affect the activity of CBP/p300 through interaction domains that are evolutionarily conserved in E1A.

Figure 1

Linear depiction of the HAdV-5 243R E1A protein. The regions conserved between serotypes are labelled as CR1, CR2 and CR4; amino-acid ordinates depict CR boundaries. The domains required for the indicated E1A functions are indicated as black bars beneath the map. The location and consensus sequences of binding motifs necessary for interaction with CBP/p300, pRb and CtBP are indicated, as are the general regions of E1A implicated in binding to the specified cellular proteins. K239, the major acetylation site targeted by CBP/p300, is also depicted.

Figure 2

Role of APC/C–CBP/p300 complexes in transcription and cell-cycle control. (A(a)) Transcription and acetylation. Cyclic AMP response element-binding protein/p300 function as transcriptional coactivators for sequence-specific DNA-binding transcription factors. These enzymes acetylate histones to alter chromatin accessibility. Cyclic AMP response element-binding protein/p300 also acetylate transcription factors to regulate their activity. Interaction with E1A or the APC/C potentially interferes and/or retargets this activity and E1A is itself a substrate for acetylation. Anaphase-promoting complex5 and/or APC7, as components of the APC/C holoenzyme, interact with CBP/p300 to stimulate inherent CBP/p300 acetyltransferase activity, and CBP/p300-dependent transcriptional activity. Early region 1A might disrupt or mimic APC/C function in this regard through its interaction with CBP/p300. (A(b)) Transcription and ubiquitylation. The recruitment of the APC/C to target promoters could potentially regulate CBP/p300 function by promoting CBP/p300 ubiquitylation. The ubiquitylation of the CBP/p300 in this instance could directly affect CBP/p300 acetyltransferase activity, and/or affect CBP/p300 interaction with other proteins, and/or promote proteasomal-mediated degradation of CBP/p300. Early region 1A could interfere with APC/C function in this regard by binding directly to CBP/p300. (B) Mitosis. The APC/C complex ubiquitylates cell-cycle regulatory proteins and targets them for proteasomal degradation. Cyclic AMP response element-binding protein functions as an E4 ligase in this regard. Whether acetylation of the APC/C by CBP/p300 regulates APC/C E3 ligase activity in this regard requires further investigation. We propose that E1A might regulate mitotic progression and/or promote genomic instability through interfering directly with APC/C function in mitosis through its ability to bind CBP/p300. Ac, acetylated-residues; pUb, polyubiquitylation; 5, 7 and 11 refer to APC/C subunits. APC11 is the functional ubiquitin ligase. P, phosphorylated residues.