Catalyzes the specific phosphorylation of arginine residues in a large number of proteins. Is part of the bacterial stress response system, where it is involved in regulating the global heat shock repressor CtsR; phosphorylates arginine residues in the winged helix-turn-helix domain of CtsR, thereby preventing its binding to DNA and consequently inducing the expression of repressed genes. 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. Acts exclusively on Arg residues, since it cannot phosphorylate Tyr, Ser, Thr, His, Asp and Lys. Has no free arginine kinase 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. Homodimer. Dimerization is important for full catalytic activity. In the 3D-structure the McsB dimer adopts a flat 'domino tile' shape, with the two active sites opening on the same side. Individual subunits are composed of the N-terminal catalytic, ATP:guanido phosphotransferase domain (PD, residues 1-263) and the C-terminal dimerization domain (DD, residues 264-355), which are linearly organized in a PD-DD-DD*-PD* manner (asterisk denotes the partner protomer). The PhK-like catalytic phosphotransferase domain is structurally adapted to target protein substrates. The C-terminal DD of McsB contains a pArg-binding pocket that allows pArg-carrying proteins to allosterically enhance McsB kinase activity. Belongs to the ATP:guanido phosphotransferase family.