The growth suppressor activities of the RB and p107 products are believed to be mediated by the reversible binding of a heterogeneous family of cellular proteins to a conserved T/E1A pocket domain that is present within both proteins. To study the functional role of these interactions, we examined the properties of cellular retinoblastoma binding protein 2 (RBP2) binding to RB, p1O7, and the related TATA-binding protein (TBP) product. We observed that although RBP2 bound exclusively to the T/E1A pocket of p107, it could interact with RB through independent T/E1A and non-T/ElA domains and with TBP only through the non-T/ElA domain. Consistent with this observation, we found that a mutation within the Leu-X-Cys-X-Glu motif of RBP2 resulted in loss of ability to precipitate p107, while RB-and TBP-binding activities were retained. We located the non-T/ElA binding site of RBP2 on a 15-kDa fragment that is independent from the Leu-X-Cys-X-Glu motif and encodes binding activity for RB and TBP but does not interact with p107. Despite the presence of a non-T/ElA binding site, however, recombinant RBP2 retained the ability to preferentially precipitate active hypophosphorylated RB from whole-cell lysates. In addition, we found that cotransfection of RBP2 can reverse in vivo RB-mediated suppression of E2F activity. These findings confirm the differential binding specificities of the related RB, p107, and TBP proteins and support the presence of multifunctional domains on the nuclear RBP2 product which may allow complex interactions with the cellular transcription machinery.An important step toward defining a mechanism underlying tumor suppressor activity of the Rb gene was the observation that the transforming products of adenovirus (ElA), simian virus 40 (large T), and human papillomavirus (E7) could precipitate wild-type RB protein (8,16,57,59). This, in turn, led to the identification of a family of cellular proteins that can reversibly bind to a discrete domain on RB, referred to as the T/ElA pocket by using the same specificity as the viral products (10, 12, 33-35, 39, 51, 52, 54). The subsequent observation that protein binding was inhibited following RB phosphorylation in the late G1 phase of the cell cycle suggested the hypothesis that RB, as well as the related p107 product, may regulate the functional activity of its binding partners by a cell cycle-dependent pattern of physical association (19,48,53).The binding domains within the RB protein have been delineated by a series of in vitro and naturally occurring mutants as two noncontiguous regions designated domains A and B which are interrupted by a spacer sequence (29,30,32) (Fig. 1A). This structure generates a hypothetical pocket conformation that (i) is conserved among a family of E1A-binding proteins, including RB, p107, and p130 (19,26,40,45); (ii) demonstrates binding to a series of viral and cellular proteins which can be blocked by short peptides containing a leucine-X-cysteine-X-glutamic acid motif (LXCXE, where X represents any amino acid); (iii) can ...