Mol. Cell. 54, 1034–1041 (2014)

Credit: ELSEVIER

FGF signaling promotes the differentiation of mouse embryonic stem cells into primary endoderm through activation of downstream adaptor proteins such as Grb2 and Sos1, which stimulate the Ras-ERK pathway. Loss of Grb2 activity results in the maintenance of pluripotency. Grb2 is part of a protein network that interacts via its SH2 domain with many different tyrosine phosphorylated proteins during stem cell differentiation. However, determining which of its numerous interactions are sufficient for differentiation remained unclear. Yasui et al. applied the previously described affinity clamp approach to develop synthetic variants of the Grb2 SH2 domain that bound specifically to one single phosphotyrosine motif. They identified three SH2 domain variants, which they called pY-clamps, that respectively bound a phosphotyrosine motif of the tyrosine phosphatase Ptpn11 (Y580), the scaffold protein Shc1 (Y239/40) and the oncoprotein Bcr (Y177). Notably, the authors found that the chimeric Grb2 containing the Ptpn11 pY-clamp and not those with the other pY-clamps was sufficient to induce primary endoderm differentiation in a Grb2-deficient background. Further studies verified that upstream FGF signaling promoted the phosphorylation of Ptpn11 at Y580, which interacts with the Grb2 SH2 domain to activate ERK signaling. Future applications of this rescue-of-function approach with affinity clamps may reveal additional specific SH2-phosophotyrosine interactions that mediate unique biological processes.