Divergence in the plasminogen-binding group a streptococcal M protein family: functional conservation of binding site and potential role for immune selection of variants.
Group A streptococci (GAS) display receptors for the human zymogen plasminogen on the cell surface, one of which is the plasminogen-binding group A streptococcal M protein (PAM). Characterization of PAM genes from 12 GAS isolates showed significant variation within the plasminogen-binding repeat motifs (a1/a2) of this protein. To determine the impact of sequence variation on protein function, recombinant proteins representing five naturally occurring variants of PAM, together with a recombinant M1 protein, were expressed and purified. Equilibrium dissociation constants for the interaction of PAM variants with biotinylated Glu-plasminogen ranged from 1.58 to 4.99 nm. Effective concentrations of prototype PAM required for 50% inhibition of plasminogen binding to immobilized PAM variants ranged from 0.68 to 22.06 nm. These results suggest that although variation in the a1/a2 region of the PAM protein does affect the comparative affinity of PAM variants, the functional capacity to bind plasminogen is conserved. Additionally, a potential role for the a1 region of PAM in eliciting a protective immune response was investigated by using a mouse model for GAS infection. The a1 region of PAM was found to protect immunized mice challenged with a PAM-positive GAS strain. These data suggest a link between selective immune pressure against the plasminogen-binding repeats and the functional conservation of the binding domain in PAM variants.
Sanderson-Smith
Martina
M
School of Biological Sciences, University of Wollongong, Wollongong, New South Wales 2522, Australia.
Batzloff
Michael
M
Sriprakash
Kabada S
KS
Dowton
Mark
M
Ranson
Marie
M
Walker
Mark J
MJ
D020816:Amino Acid Motifs; D000818:Animals; D000942:Antigens, Bacterial / Q000737:chemistry* / Q000378:metabolism; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D019921:Biotinylation; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism; D017931:DNA Primers / Q000737:chemistry; D016521:Electrophoresis, Gel, Pulsed-Field; D004591:Electrophoresis, Polyacrylamide Gel; D019143:Evolution, Molecular; D014644:Genetic Variation; D007107:Immune System; D007700:Kinetics; D051379:Mice; D015233:Models, Statistical; D010455:Peptides / Q000737:chemistry; D010802:Phylogeny; D010957:Plasmids / Q000378:metabolism; D010958:Plasminogen / Q000737:chemistry*; D016133:Polymerase Chain Reaction; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry; D017422:Sequence Analysis, DNA; D013291:Streptococcus / Q000378:metabolism*; D013696:Temperature
D000942:Antigens, Bacterial; D001425:Bacterial Outer Membrane Proteins; D002352:Carrier Proteins; D017931:DNA Primers; D010455:Peptides; D011994:Recombinant Proteins; C014689:streptococcal M protein; D010958:Plasminogen