Crystal packing induces a conformational change in profilin-I from Acanthamoeba castellanii

Profilin-I from Acanthamoeba castellanii is a 13-kDa protein that binds actin and poly-l-proline. The native protein has been crystallized in two different but closely related forms. The second form proved more amenable to three-dimensional structural determination using heavy-atom isomorphous methods to obtain crystallographic phase information. We used the second crystal structure as a test molecule in the molecular replacement procedure to determine the structure of the first crystal form of profilin-I. More residues participate in crystal lattice contacts in the first crystal form than in the second. The two crystal forms differ significantly in the C-terminal helix that interacts with actin and in the loop preceding this helix. Coordinates of some main chain atoms here differ by about 1.0 A, and side chain atoms differ by more than 2.0 A.