Molecular and cellular analysis of Grb2 SH3 domain mutants: interaction with Sos and dynamin.

Quantitative analysis of Grb2/dynamin interaction through plasmon resonance analysis (BIAcore) using Grb2 mutants showed that the high affinity measured between Grb2 and dynamin is essentially mediated by the N-SH3 domain of Grb2. In order to study the interactions between Grb2 and either dynamin or Sos in more detail, Grb2 N-SH3 domains containing different mutations have been analysed. Two mutations were located on the hydrophobic platform binding proline-rich peptides (Y7V and P49L) and one (E40T) located in a region that we had previously shown to be essential for Grb2/dynamin interactions. Through NMR analysis, we have clearly demonstrated that the structure of the P49L mutant is not folded, while the other E40T and Y7V mutants adopt folded structures that are quite similar to that described for the reference domain. Nevertheless, these point mutations were shown to alter the overall stability of these domains by inducing an equilibrium between a folded and an unfolded form. The complex formed between the peptide VPPPVPPRRR, derived from Sos, and the E40T mutant was shown to have the same 3D structure as that described for the wild-type SH3 domain. However, the VPPPVPPRRR peptide adopts a slightly different orientation when it is complexed with the Y7V mutant. Finally, the affinity of the proline-rich peptide GPPPQVPSRPNR, derived from dynamin, for the Grb2 N-SH3 domain was too low to be analyzed by NMR. Thus, the interaction between either Sos or dynamin and the SH3 mutants were tested on a cellular homogenate by means of a far-Western blot analysis. In these conditions, the P49L mutant was shown to be devoid of affinity for Sos as well as for dynamin. The Y7V SH3 mutant displayed a decrease of affinity for both Sos and dynamin, while the E40T mutant exhibited a decrease of affinity only for dynamin. These results support the existence of two binding sites between dynamin and the Grb2 N-SH3 domain.