Elsholz et al. (2012), showed that McsB and YwlE are a protein arginine kinase and phosphatase, rather than a tyrosine kinase and phosphatase because they observed only an McsB/YwlE-dependent detection of protein arginine phosphorylation or dephosphorylation in vivo. Specifically, they suggested that YwIE acts as a PAP in vivo.
McsB and YwlE were thought to be tyrosine kinases and phosphatases. However,  in 2012, Elsholz et al. detected 121 arginine phosphorylation sites in 87 proteins in living Bacillus Subtilis (B.subtillis), a gram-positive bacterium present in soil and human gastrointestinal tract. Their observations led them to believe that protein arginine phosphorylation exists in vivo as a posttranslational modification in bacteria. The arginine-phosphorylated proteins they detected were distributed among "distinct physiological classes of proteins" such as regulators, metabolic enzymes, stress, and ribosomal proteins. This result suggested that YwlE acts as a protein arginine phosphatase that explicitly dephosphorylates arginine residues both in vitro and in vivo
Secondly, Elsholz et al. (2012) were only able to detect protein arginine phosphorylation in a YwIE mutant gene and not the wild-type strain. But protein phosphorylates on either serine, threonine, or tyrosine were detected in both wild-type and a YwIE mutant strain in equal amounts. Therefore, they thought that YwIE might solely act as a protein arginine phosphatase. That is, the detection of protein arginine phosphorylation depended on the presence of YwIE. They confirmed this hypothesis after failing to detect protein arginine phosphorylation after (1) analyzing a mutant extract treated in vitro with purified YwIE protein before conducting mass spectroscopy analysis; and (2) overexpressing the YwIE in trans in a YwIE mutant in-vivo. The close interaction of the arginine phosphorylated proteins with YwIE suggested that the stability of the modifications was indeed influenced by the YwIE protein.