2.5 A resolution crystal structure of the motile major sperm protein (MSP) of Ascaris suum.

We have determined the structure of the Ascaris major sperm protein (MSP) to 2.5 A resolution using X-ray crystallography. The MSP polypeptide chain has an immunoglobulin-like fold based on a seven-stranded beta sandwich. In two strands, cis-proline residues impart distinctive kinks, and overall the structure most closely resembles that of the N-terminal domain of the bacterial chaperonin, PapD. In the C2 crystal form which we have solved here, two MSP chains are tightly associated in the asymmetric unit and are related by a non-crystallographic 2-fold rotation axis. This arrangement almost certainly represents the MSP dimer that is present in solution. Additionally, the arrangement of two MSP dimers at one of the crystallographic 2-fold axes in the 215 A unit cell suggests a possible mode for the assembly of MSP into the filaments which promote cell movement. This dimer-dimer association is based on a beta sheet extension mechanism between adjoining MSP monomers which resembles the interaction between PapD and its protein substrate.