Characterization of Plasminogen-Binding Group A Streptococcal M Proteins from the Aspects of Function, Structure, and Evolution

Doctoral Dissertation
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Abstract

Group A streptococcus (GAS) is one of leading infectious causes of human morbidity and mortality. In particular, pattern D GAS isolates express plasminogen-binding group A streptococcal M protein (PAM) that utilizes its A-domain to specifically capture the K2 domain in human plasminogen (hPg). The binding to PAM facilitates activation of hPg by GAS-derived streptokinase on the cell surface. Plasmin generated degrades fibrin encapsulating bacterial cells, which is one of the major pathogenesis mechanisms employed by skin-tropic pattern D strains to escape the host immune defense.

Seven recombinant PAMs from different GAS strains were investigated in this work. Each PAM dimerizes in solution at 25 °C, wherein the C-domain constitutes the most important region in PAM dimerization. NMR solution structures of peptides that contain the A- and B-domains demonstrated that both domains are helix-destabilizing, and thus contribute little to dimerization. A structural model was accordingly developed to describe the non-ideal dimerization pattern of PAM. It is noteworthy that dimeric PAMs dissociate into unstructured monomers at 37 °C, to different extents. Although all PAMs tightly bind to hPg in a nM range at 25 °C, the reduction of α-helical content at 37 °C makes class II PAMs that only contain the a2-repeat interact with hPg ~1,000 times weaker, but impacts little to hPg-binding of class I/III PAMs containing complete a1a2-repeats. It is further corroborated that the a1-repeat protects the α-helix in the a2-repeat, making binding relevant residues in the latter repeat optimally orientate. However, this protection disappears when the a1-repeat is absent in class II PAMs. Another divergence among these PAMs is that dimeric class I/III PAMs completely dissociate once binding to excessive K2hPg at 25 °C, while monomeric and dimeric species coexist in the case of class II PAMs.

NMR structures of two complexes, AGL55NS88.2-K2hPg and KTI55SS1448-K2hPg, in which AGL55NS88.2 and KTI55SS1448 are truncated from the corresponding class II PAMs, elucidate the atom-level mechanisms underlying the differences in hPg-binding between class II and class I/III PAMs. On the basis of complex structures and the phylogeny of PAMs, the α-helical content in the A-domain is discovered to be a driving force in the PAM evolution.

Attributes

Attribute NameValues
Author Cunjia Qiu
Contributor Paul W. Huber, Committee Member
Contributor Holly V. Goodson, Committee Member
Contributor Francis J. Castellino, Research Director
Contributor Shaun W. Lee, Committee Member
Degree Level Doctoral Dissertation
Degree Discipline Chemistry and Biochemistry
Degree Name Doctor of Philosophy
Banner Code
  • PHD-BCHM

Defense Date
  • 2019-11-21

Submission Date 2019-12-02
Subject
  • Biophysics

  • Microbiology

  • Protein Interactions

  • NMR Spectroscopy

Record Visibility Public
Content License
  • All rights reserved

Departments and Units
Catalog Record

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