The structure of the Gemella haemolysans M26 IgA1 protease trypsin-like domain.

Immunoglobulin A (IgA) proteases are a group of bacterial-derived enzymes that selectivity hydrolyze human IgA in the hinge region that is unique to this immunoglobulin. Several IgA protease (IgAP) families have evolved this ability using both metalloprotease and serine protease chemical mechanisms. One family of metal-dependent IgAPs is the M26 family. This family can be grouped into two subfamilies based upon the presence or absence of a trypsin-like domain found N-terminal to the IgAP domain. The role of this domain in IgAP structure and function is poorly understood. Here, we present the first structural characterization of an M26 IgAP trypsin-like domain from Gemella haemolysans (GhTrp). These structural data demonstrate that the GhTrp domain possesses a trypsin-like fold but contains significant deviations in the surface-loop structure that is known to be coupled to protease selectivity. The lack of observable catalytic function coupled with the structural data suggest that this domain may exist in a pro-enzyme-like state that can potentially be activated when the domain is N-terminally proteolytically excised from the larger M26 IgAP structure.