Post-translational modifications of pRB. The open reading frame of pRB is depicted with the A and B regions of the pocket indicated. Cyclin/Cdk phosphorylation sites that have been investigated by mutagenesis are shown as are sites of phosphorylation following DNA damage. The location of acetylated and sumoylated lysines are indicated. Arrows indicate the approximate location of Cdk4 and cyclin E binding sites that are needed for optimal phosphorylation of pRB. Caspase cleavage of pRB occurs on the C-terminal side of aspartate 886, removing amino acids 887 to 928.