Catalyzes the specific phosphorylation of arginine residues in proteins (By similarity). Is required for stress tolerance and virulence in S.aureus. Acts as a modulator of the repressor activity of CtsR upon heavy metal and oxidative stress caused by these metal ions (PubMed:22902728). ATP + L-arginyl-[protein] = ADP + H(+) + N(omega)-phospho-L-arginyl-[protein] Interacts with McsA and CtsR. Up-regulated by heavy metals such as Cu(2+) and Cd(2+), but Zn(2+) and Co(2+) have no effect. Forms part of an operon with ctsR, mcsA and clpC, which is repressed by CtsR. Hypersensitivity to heavy metal stress, temperature stress, osmotic pressure stress and oxidative stress. Deletion of mcsB also results in sensitivity to variations in pH and lowered expression of the clpC operon under adverse extracellular conditions. Additionally, virulence traits such as hemolytic activity, proteolysis, biofilm formation, and evasion from peritoneal fluid killing are substantially reduced in cells lacking this gene. Interestingly, mutant cells also show a significant reduction in expression of virulence determinants hla and saeS. Belongs to the ATP:guanido phosphotransferase family.