THE AMINO-TERMINAL REGION OF THE RETINOBLASTOMA GENE-PRODUCT BINDS A NOVEL NUCLEAR MATRIX PROTEIN THAT COLOCALIZES TO CENTERS FOR RNA PROCESSING

The tumor suppressing capacity of the retinoblastoma protein (p110(RB)) is dependent on interactions made with cellular proteins through its carboxy-terminal domains. How the p110(RB) aminoterminal region contributes to this activity is unclear, though evidence now indicates it is important for both growth suppression and regulation of the full-length protein. We have used the yeast two-hybrid system to screen for cellular proteins which bind to the first 300 amino acids of p110(RB). The only gene isolated from this screen encodes a novel 84-kD nuclear matrix protein that localizes to subnuclear regions associated with RNA processing. This protein, p84, requires a structurally defined domain in the amino terminus of p110(RB) for binding. Furthermore, both in vivo and in vitro experiments demonstrate that p84 binds preferentially to the functionally active, hypophosphorylated form of p110(RB). Thus, the amino terminus of p110(RB) may function in part to facilitate the binding of growth promoting factors at subnuclear regions actively involved in RNA metabolism.