The mouse beta PIX-SH3 domain, residues 8-63 of P21-activated kinase interacting exchange factor, has been characterized by X-ray diffraction. Crystals belonging to space group P3(2)21 diffracted to 2.0 A and the structure was phased by the single-wavelength anomalous diffraction method. The domain is a compact beta-barrel with all overall conformation similar to the general SH3 structure. The X-ray structure shows Mouse beta PIX-SH3 domain binding the way in which the PPIX characteristic amino acids do so for ail unconventional ligand binding surface. This arrangement provides a rationale For the unusual ligand recognition motif exhibited by mouse beta PIX-SH3 domain. Comparison with mother SH3/peptide complex shows that the recognition mode of the mouse beta PIX-SH3 domain should be very similar to the RXXK ligand binding mode. The unique large and planar hydrophobic pocket may contribute to the promiscuity of beta PIX-SH3 domain resulting in its multiple biological functions. (c) 2005 Elsevier Inc. All rights reserved.