Component of the pilus tip. Can bind covalently, via its two reactive thioester bonds, to molecular targets from host cell surface and can thus mediate adhesion of the streptococcal pili to host cells. Lysine side chains or a carbohydrate with a free amine group might be candidates for Cpa binding. In vitro, can covalently bind to spermidine, but it is unlikely that spermidine is the natural target of Cpa. Monomer. Proteolytically processed and assembled in pili through a transpeptidation reaction catalyzed by a sortase, which leads to a covalent link between Cpa and a major pilin subunit.