Bacterial surface-associated plasmin formation is believed to contribute to invasion, although the underlying molecular mechanisms are poorly understood. To define the components necessary for plasmin generation on group A streptococci we used strain AP53 which exposes an M-like protein ("PAM") that contains a plasminogen-binding sequence with two 13-amino acid residues long tandem repeats (a1 and a2). Utilizing an Escherichia coli-streptococcal shuttle vector, we replaced a 29-residue long sequence segment of Arp4, an M-like protein that does not bind plasminogen, with a single (a1) or the combined a1a2 repeats of PAM. When expressed in E. coli, the purified chimeric Arp/PAM proteins both bound plasminogen, as well as plasmin, and when used to transform group A streptococcal strains lacking the plasminogen-binding ability, transformants with the Arp/PAM constructs efficiently bound plasminogen. Moreover, when grown in the presence of plasminogen, both Arp/PAM- and PAM-expressing streptococci acquired surface-bound plasmin. In contrast, plasminogen activation failed to occur on PAM- and Arp/PAM-expressing streptococci carrying an inactivated streptokinase gene: this block was overcome by exogenous streptokinase. Together, these results provide evidence for an unusual co-operation between a surface-bound protein, PAM, and a secreted protein, streptokinase, resulting in bacterial acquisition of a host protease that is likely to spur parasite invasion of host tissues.
