All ETDs from UAB

Advisory Committee Chair

Narayana Vl Sthanam

Advisory Committee Members

Terje Dokland

Todd Green

Rama Krishna

Aaron Lucius

David Pritchard

Document Type

Dissertation

Date of Award

2011

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Multi-drug resistance in Gram-positive bacteria is a growing and unwavering challenge for twenty-first century medicine. The enzyme sortase, identified just around the turn of the century, has since become a promising target for development of anti-infective drugs. Streptococcus agalactiae or Group B Streptococcus (GBS), encodes multiple sortases, which participate in various processes and whose functions are only beginning to be understood in detail. Sortases are cysteine proteases that are crucial for Gram-positive pili biogenesis. Two classes of sortases, class C and class A, participate in the assembly of structural components (pilins) to form the pili and subsequent anchoring of the pili onto the cell wall, respectively. S. agalactiae express pili on the bacterial cell surface and the PI-1 pathogenecity island of S. agalactiae SAG 2603 V/R strain encodes three pilins (shaft, tip and base pilins) and two class C sortases, sortase C1 and sortase C2. All GBS strains also encode a housekeeping sortase A. The three pilins of PI-1 islet contain a unique but subtly different sorting signal motif. Sortases C1 and C2 show functional redundancy for the major pilin but narrow specificity for the minor pilins. Sortase A, a transpeptidase, can recognize the sorting signal motifs of the shaft and the base pilins. The structural basis of this discrimination between slightly different substrate motifs by different classes of sortases is still not understood. This work presents the first structural characterization of the two classes of sortase enzymes in a single pathogen and strain. Based on the crystal structures of GBS Sortase A, Sortase C1 in two space groups and a mutant of Sortase C1, we have identified unique features in Sortase C1, which include the presence of an active-site occluding `lid', an anchor motif, a hydrophobic lid residue that protects a hydrophobic pocket in the active site. A structure of an inhibitor complex of Sortase C1 and apo-sortase C1 in a third space group give insights into the mechanism of ligand binding and accompanying conformational changes for the Sortase C1 lid. We present a model of substrate recognition, which is likely universal in all class C sortases.

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