Structure of the profilin-poly-L-proline complex involved in morphogenesis and cytoskeletal regulation

Profilin, a ubiquitous low molecular weight (13,000-15,000 M(r)) actin binding protein, regulates the formation of F-actin structures in vivo, and is localized to specific cellular regions through interaction with proline- rich sequences. Here we report the 2.2 Å X-ray structure of the complex between human platelet profilin (HPP) and a decamer of L-proline (L-Pro). The L-Pro peptide adopts a left-handed type II poly-L-proline helix (PPiI) and binds to a highly conserved patch of aromatic amino acids on the surface of profilin. The peptide and actin binding sites reside on orthogonal surfaces, and L-Pro binding does not result in a conformational rearrangement of HPP. This structure suggests a mechanism for the localization of profilin and its actin-related activities to sites of actin filament assembly in vivo.