Besides CRK, FRS2 also binds GRB2, the cyclin-dependent kinase substrate p13(SUC1), and the SH3 domain of SRC. There is also evidence for a C3G/CRK/SHP2/GAB2 complex, which is trafficked to the endosome, where C3G interacts with RAP1, triggering sustained RAP1 activation and prolonged B-RAF/MEK1/MAPK signalling. Crk-L is the predominant CRK isoform that interacts with C3G in several cell types; it is abundant in PC12 cells. PC12 cells also express high levels of Crk-II and low, but detectable, levels of Crk-I. Activation of Elk-1 by NGF was potently increased by cotransfection of exogenous Crk-II and Crk-L, but only weakly by Crk-I. In the absence of NGF, the expression of CRK isoforms activated Elk-1 minimally.