Structural and Functional Characterization of a Modular Immunoglobulin A Protease from Thomasclavelia ramosa

Abstract

Immunoglobulin A proteases (IgAPs) are a diverse group of enzymes secreted from a wide range of mucosal bacteria. These enzymes have convergently evolved to cleave immunoglobulin A (IgA), the main antibody found on the mucosa, as a means of modulating the bacterium’s relationship with their host tissues. Due to the various biological functions and biochemical properties of these enzymes, the study of IgAPs has provided multifaceted insight into aspects of mucosal immunity, enzyme structure and function, and the structural basis for substrate specificity.

Only two of three known IgAP enzyme families have been investigated using an in-depth structural and functional approach. This thesis thus aimed to carry out these analyses on the IgAP from Thomasclavelia ramosa as a representative member of this last poorly characterized family. X-ray crystallographic, small-angle X-ray scattering, and gel-based kinetic techniques were used to reveal that, unlike the other two IgAP families, the T. ramosa IgAP has a truly modular protein architecture that can be split into and produced as distinct minimized domains that retain function. The crystal and solution-scattering structures of various domain constructs were also used to generate a working model for how the T. ramosa IgAP recognizes and has high specificity for IgA. This thesis provides the first in-depth biochemical account of this IgAP family and pave the way for advances in clinically relevant IgAP-related research and our understanding of IgAPs as a whole.