Catalyzes fatty acylation of the protoxin (RtxA) at internal lysine residues, thereby converting it to the active toxin. a fatty acyl-[ACP] + L-lysyl-[protein] = H(+) + holo-[ACP] + N(6)-(fatty acyl)-L-lysyl-[protein] Cells lacking RtxC do not show reduced toxicity (PubMed:19900531). However, the RtxA toxin displays reduced actin cross-linking activity (PubMed:19900531). Belongs to the RTX toxin acyltransferase family.