Crystal structure of a Src-homology 3 (SH3) domain

THE Src-homologous SH3 domain is a small domain present in a large number of proteins that are involved in signal transduction, such as the Src protein tyrosine kinase, or in membrane-cyto-skeleton interactions, but the function of SH3 is still unknown (reviewed in refs 1-3). Here we report the three-dimensional struc-ture at 1.8 Å resolution of the SH3 domain of the cytoskeletal protein spectrin expressed in Escherichia coli. The domain is a compact β-barrel made of five antiparallel β-strands. The amino acids that are conserved in the SH3 sequences are located close to each other on one side of the molecule. This surface is rich in aromatic and carboxylic amino acids, and is distal to the region of the molecule where the N and C termini reside and where SH3 inserts into the α-spectrin chain. We suggest that a protein ligand binds to this conserved surface of SH3.