Acetylation can regulate cell-cycle progression

Abstract

The retinoblastoma tumour suppressor (pRb) is crucial for the negative control of cell proliferation. The protein has a major role in G1- to S-phase transition by controlling key transcription factors such as the E2F family. In tumour cells, pRb is sequestered by viral oncoproteins (E1A) and regulated by phosphorylation through G1 cyclin-dependent kinases (CDKs),primarily cyclinD/CDK4 and cyclinE/Cdk2,which sequentially phosphorylate pRb as the cells move towards S-phase.In a new study, Chan et al. 1xAcetylation control of the retinoblastoma tumour-suppressor protein. Chan, H.M. et al. Nat. Cell Biol. 2001; 3: 667–674CrossRef | PubMed | Scopus (194)See all References1 have identified acetylation as a new type of regulator of pRb function. The acetylation is mediated by p300 through adenovirus E1A recruitment of pRb and p300/CREB binding protein (CBP) in a multimeric complex. Both pRb and p300/CBP are targets for viral oncoproteins, such as the adenovirus E1A.In an elegant series of experiments, the authors tested the acetylation of pRb by the histone acetyl transferase (HAT) activity of p300 using a glutathione S-transferase (GST)–pRb fusion protein in vitro. This is influenced by the integrity of a pocket domain at the C-terminus of the pRb protein. Two pocket mutants, isolated from tumour cells, showed reduced acetylation. To establish whether this also happened in vivo, the authors used an antibody against acetylated lysines in pRB and showed the presence of endogenous acetylated pRb. Under similar circumstances, the pocket mutant was not acetylated. Use of mutant derivatives showed that the main site of acetylation was located between amino-acid residues 794 and 880. There are eight lysine residues in this region, and Chan et al. focused on the five lysines between residues 830 and 884, systematically altering each lysine to arginine and identifying lysines 873 and 874 as being acetylated. Having done this, the authors assessed whether acetylation might influence pRb phosphorylation, by using constructs where the lysines at residues 873 and 874 were replaced by arginines or glutamines (873/874RR and 873/874QQ, respectively). A change to glutamine mimics the acetylation status, whereas a change to arginine prevents acetylation. Experiments using these constructs showed that wild-type pRb and 873/874 RR were phosphorylated to a similar extent, but the 873/874 QQ construct showed a significant reduction in phosphorylation. The p300-dependent acetylation of pRb is increased by adenovirus E1A, through recruitment of p300 and pRb into the same complex. This process is dependent on E1A concentrations: at high E1A concentrations the acetylation of pRb is lost. To assess the function of acetylation, the authors looked at proteins that interact with the C-terminus of the pRb protein. For example, the MDM2 oncoprotein bound more efficiently to pRb in its acytelated form, both in vitro and in vivo. Similar experiments with E2F failed to detect any differences.These recent advances suggest acetylation as a new control mechanism in regulating pRb activity and a new mechanism through which viral oncoproteins can affect tumour-suppressor activity.