CRYSTAL STRUCTURE OF THE SH2 DOMAIN OF THE PROTEIN SHC: pH-DEPENDENT SELF-ASSOCIATION. Klaus Fütterer, Stéphane Réty, Richard A. Grucza, Gabriel Waksman, Washington University School of Medicine, Department of Biochemistry and Molecular Biophysics, Saint Louis, MO 63110, USA.

The Src Homologous and Collagen-like (SHC) protein plays an essential role in signal transduction pathways leading to the activation of the protein Ras. SHC contains a Src Homology 2 (SH2) domain, the function of which is to specifically recognize tyrosine phosphorylated peptide ligands. The structure of an SH2 domain can be described as a central [[beta]]-sheet flanked by two [[alpha]]-helices (G. Waksman et al., J. Kuriyan, 1992, Nature 358: 646-653). The peptide binding surface of SH2 domains in general consists of two binding pockets on either side of the central [[beta]]-sheet. While the binding pocket for the phosphotyrosine is conserved, its counterpart accommodating the third amino acid C-terminal to the phosphotyrosine varies in shape and depth between different SH2 domains, reflecting specificity for different peptide ligands.

This contribution presents the crystal structure of the SH2 domain of SHC determined by multiple isomorphous replacement at a resolution of 2.5 Å (S. Réty et al., G. Waksman, 1996, Protein Science, in press). The SH2 domain of SHC reveals a fold similar to other SH2 domains. The peptide binding surface of SHC-SH2 resembles that of the SH2 domain of Src (G. Waksman et al., J.Kuriyan, 1993, Cell 72: 779-790) in that it presents a clearly delineated pocket for the third residue C-terminal to the phosphotyrosine, but it is deeper and more distinct in SHC than in Src.

A novel feature of the SH2 domain of SHC is the observation of an intermolecular disulfide bond accompanied by an extensive dimer interface between two symmetry-related molecules. The potential dimerization of SHC-SH2 in solution was studied under reducing conditions using analytical ultracentrifugation and polyacrylamide gel electrophoresis. The results suggest that the SH2 domain of SHC dimerizes in a pH-dependent manner, where low pH conditions (~pH 4.5) are conducive to dimer formation. Dimerization of SHC may have important biological implications, possibly promoting the assembly of large heteromultimeric signaling complexes.