Molecular Analysis of Phosphorylated Tyrosine-Binding Proteins in the Transduction of Proliferation and Differentiation Signals

Abstract

The human CRK protein consists of an SH2 region that binds to phosphotyrosine-containing peptides and an SH3 region which associates with cellular factors to modulate transformation. The microinjection of CRK protein induced neurite formation in PC12, a rat pheochromocytoma cell line. This activity was abolished by mutation of the CRK protein at either SH2 or SH3, and was suppressed by monoclonal antibodies against CRK SH2 or p21ras. This set of observations suggests that both the SH2 and the SH3 domains of human CRK protein are required for neuronal differentiation of PC12 cells, and that they function upstream of Ras. In addition, from the search for proteins that bind to the SH3 domain of CRK and activate Ras, we identified a new guanine nucleotide-releasing protein which was designated C3G. The nucleotide sequence of the 4.1-kb C3G cDNA contains a 3.2-kb open reading frame that encodes a 121-kDa protein. The mRNAs of both the C3G and the CRK proteins are expressed ubiquitously in adult and fetal human tissues. The overall results of this investigation suggest that the complex of CRK and C3G may transduce signals from tyrosine kinases to Ras in a number of different tissues.