Mechanism of cell surface expression of the Streptococcus mitis platelet binding proteins PblA and PblB.
PblA and PblB are prophage-encoded proteins of Streptococcus mitis strain SF100 that mediate binding to human platelets. The mechanism for surface expression of these proteins has been unknown, as they do not contain signal sequences or cell wall sorting motifs. We therefore assessed whether expression of these proteins was linked the lytic cycle of the prophage. Deletion of either the holin or lysin gene resulted in retention of PblA and PblB in the cytoplasm, and loss of these proteins from the cell wall. Flow cytometric analysis revealed that induction of phage replication in SF100 produced a subpopulation of cells with increased permeability. This effect was abrogated by disruption of the holin and lysin genes. Treatment of these mutants with exogenous PblA and PblB restored surface expression, apparently via binding of the proteins to cell wall choline. Loss of PblA and PblB expression was associated with decreased platelet binding in vitro, and reduced virulence in an animal model of endocarditis. Thus, expression of PblA and PblB occurs via a novel mechanism, whereby phage induction increases bacterial permeability and release of the proteins, followed by their binding to surface of viable cells. This mechanism may be important for endovascular infection.
Mitchell
Jennifer
J
Veterans Affairs Medical Center and the University of California, San Francisco, CA 94121, USA.
Siboo
Ian R
IR
Takamatsu
Daisuke
D
Chambers
Henry F
HF
Sullam
Paul M
PM
D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001792:Blood Platelets / Q000378:metabolism*; D015153:Blotting, Western; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002473:Cell Wall / Q000737:chemistry / Q000378:metabolism; D002794:Choline / Q000737:chemistry / Q000378:metabolism; D004696:Endocarditis / Q000378:metabolism / Q000382:microbiology; D004798:Enzymes / Q000235:genetics / Q000502:physiology; D004983:Ethanolamines / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D008854:Microscopy, Electron; D015394:Molecular Structure; D009154:Mutation; D039002:Prophages / Q000235:genetics / Q000254:growth & development / Q000648:ultrastructure; D011485:Protein Binding; D011817:Rabbits; D017108:Streptococcus Phages / Q000235:genetics / Q000254:growth & development / Q000648:ultrastructure; D034361:Streptococcus mitis / Q000378:metabolism* / Q000472:pathogenicity / Q000821:virology; D014466:Ultraviolet Rays; D014764:Viral Proteins / Q000235:genetics / Q000502:physiology; D014774:Virulence / Q000235:genetics; D014775:Virus Activation / Q000528:radiation effects
D001426:Bacterial Proteins; D002352:Carrier Proteins; D004798:Enzymes; D004983:Ethanolamines; D014764:Viral Proteins; C028806:lysin, streptococcus bacteriophage; D002794:Choline