Toxin component of the prophage tail-derived protein translocation system Afp, which is the causative agent of enteric redmouth disease in salmonid fish species. Mono-O-GlcNAcylates the small GTPase RhoA in eukaryotic host cells at Tyr-34, using UDP-N-acetylglucosamine (UDP-GlcNAc) as the sugar donor. Glycosylation of RhoA results in impaired effector and regulator interaction and inactivation of downstream RhoA signaling which leads to actin filament depolymerization and blocks cytokinesis and gastrulation during zebrafish embryo development. To a lesser extent, is also able to glycosylate other Rho family GTPases (RhoB, RhoC, Rac1, Rac2, Rac3, and Cdc42) in vitro at a switch I tyrosine residue, but not Ras proteins. L-tyrosyl-[protein] + UDP-N-acetyl-alpha-D-glucosamine = H(+) + O-(N-acetyl-alpha-D-glucosaminyl)-L-tyrosyl-[protein] + UDP Is likely secreted by the antifeeding prophage tail (Afp) protein translocation system into eukaryotic host cell. Targets the host cell membrane, and shows co-localization with RhoA. The glycosyltransferase domain is the Afp18 protein domain mediating toxicity.