Cartoon representations of the Spc-SH3 domain. A) X-ray structure of WT Spc-SH3 (PDB code: 1SHG). Secondary structures are attributed according to the DSSP program (β-strands: yellow; 310 helix: red; loops: green) and labelled according to the generally adopted nomenclature for SH3 domains. The three mutated residues are shown as blue sticks. B) Structure of the R21A Spc-SH3:p41 complex (PDB code: 2JMA), in blue, superimposed to the WT Spc-SH3 structure in grey. Residues that interact with the ligand are shown as sticks. The structure of p41 is shown in cyan.