NECDIN, A POSTMITOTIC NEURON-SPECIFIC GROWTH SUPPRESSOR, INTERACTS WITH VIRAL TRANSFORMING PROTEINS AND CELLULAR TRANSCRIPTION FACTOR E2F1

Necdin is a nuclear protein expressed in virtually all postmitotic neu rons, and ectopic expression of this protein strongly suppresses the p roliferation of NIH3T3 cells, Simian virus 40 large T antigen targets both p53 and the retinoblastoma protein (Rb) for cellular transformati on, By analogy with the interactions of the large T antigen with these nuclear growth suppressors, we examined the ability of necdin to bind to the large T antigen, Necdin was co-immunoprecipitated with the lar ge T antigen from the nuclear extract of necdin cDNA-transfected COS-1 cells, Yeast two-hybrid and in vitro binding analyses revealed that n ecdin bound to an amino-terminal region of the large T antigen, which encompasses the Rb-binding domain, Moreover, necdin bound to adenoviru s E1A, another viral oncoprotein that forms a specific complex with Rb , We then examined the ability of necdin to bind to the transcription factor E2F1, a cellular Rb-binding factor involved in cell-cycle progr ession, Intriguingly, necdin, like Rb, bound to a carboxyl-terminal do main of E2F1, and repressed E2F-dependent transactivation in vivo, In addition, necdin suppressed the colony formation of Rb-deficient SAOS- 2 osteosarcoma cells. These results suggest that necdin is a postmitot ic neuron-specific growth suppressor that is functionally similar to R b.