Catalyzes the specific phosphorylation of arginine residues in a large number of proteins. Is part of the bacterial stress response system. Protein arginine phosphorylation has a physiologically important role and is involved in the regulation of many critical cellular processes, such as protein homeostasis, motility, competence, and stringent and stress responses, by regulating gene expression and protein activity. ATP + L-arginyl-[protein] = ADP + H(+) + N(omega)-phospho-L-arginyl-[protein] Appears to be allosterically activated by the binding of pArg-containing polypeptides to the pArg-binding pocket localized in the C-terminal domain of McsB. Belongs to the ATP:guanido phosphotransferase family.