Dimerization of the docking/adaptor protein HEF1 via a carboxy-terminal helix-loop-helix domain

HEF1, p130(Cas), and Efs define a family of multidomain docking proteins wh ich plays a central coordinating role for tyrosine-kinase-based signaling r elated to cell adhesion. HEF1 function has been specifically implicated in signaling pathways important for cell adhesion and differentiation in lymph oid and epithelial cells. While the SH3 domains and SH2-binding site domain s (substrate domains) of HEF1 family proteins are well characterized and bi nding partners known, to date the highly conserved carboxy-terminal domains of the three proteins have lacked functional definition. In this study, we have determined that the carboxyterminal domain of HEF1 contains a diverge nt helix-loop-helix (HLH) motif. This motif mediates HEF1 homodimerization and HEF1 heterodimerization with a recognition specificity similar to that of the transcriptional regulatory HLH proteins Id2, E12, and E47. We had pr eviously demonstrated that the HEF1 carboxyterminus expressed as a separate domain in yeast reprograms cell division patterns, inducing constitutive p seudohyphal growth. Here we show that pseudohyphal induction by HEF1 requir es an intact HLH, further supporting the idea that this motif has an effect or activity for HEF1, and implying that HEF1 pseudohyphal activity derives in part from interactions with yeast helix-loop-helix proteins. These combi ned results provide initial insight into the mode of function of the HEF1 c arboxy-terminal domain and suggest that the HEF1 protein may interact with cellular proteins which control differentiation. (C) 1999 Academic Press.