GCIP, a novel human Grap2 and cyclin D interacting protein, regulates E2F-mediated transcriptional activity

Regulation of mammalian cell growth and proliferation is governed through r eceptor-mediated signaling networks that ultimately converge on the cell cy cle machinery. Adaptor proteins play essential roles in the formation of in tracellular signaling complexes, relaying extracellular signals from the pl asma membrane to the nucleus of a cell, The leukocyte-specific adaptor prot ein Grap2 is a central linker protein in immune cell signaling and activati on Using Grap2 as bait protein, we identified a novel human protein, GCIP ( Grap2 cyclin-D interacting protein.), We found that GCIP bound to Grap2 in both yeast two-hybrid assays and in mammalian cells through binding to the COOH-terminal unique domain and SH3 domain (designated QC domain) of Grap2, GCIP also associated with cyclin D both in vitro and in vivo. The expressi on of GCIP was found in all human tissues examined with the highest level o f expression in the heart, muscle, peripheral blood leukocytes, and brain. Furthermore, phosphorylation of retinoblastoma protein by cyclin D-dependen t protein kinase was reduced and E2F1-mediated transcription activity was i nhibited in cells transfected with GCIP. High level expression of GCIP in t erminally differentiated tissues and the inhibition of E2F1 transcription a ctivation suggest that GCIP could play an important role in controlling cel l differentiation and proliferation.