Functions as a protein modifier covalently attached to lysine residues of substrate proteins. The protein modification process is termed sampylation and involves the formation of an isopeptide bond between the SAMP3 C-terminal glycine carboxylate and the epsilon-amino group of lysine residues on target proteins. Seems to be able to form polymeric chains with itself at Lys-18, Lys-55 and Lys-62, similar to ubiquitin and other ubiquitin-like proteins. SAMP3 appears not to serve as a proteolytic signal in the cell to target proteins for degradation by proteasomes. May regulate molybdenum cofactor (MoCo) biosynthesis by inhibiting the activity of MPT synthase MoaE under aerobic conditions, providing a hierarchy of oxygen use prior to that of alternative electron acceptors such as DMSO. Monomer. Up-regulated by the addition of dimethyl sulfoxide to aerobically growing cells. These conditions also increase the levels of SAMP3 conjugates in cells. The C-terminal glycine is likely acyl-adenylated (-COAMP) by UbaA. Cells lacking this gene grow similarly to wild-type in the presence of either DMSO or oxygen as the terminal electron acceptor; thus, SAMP3 is not needed for DMSO respiration, suggesting that it is not required for molybdenum cofactor (MoCo) biosynthesis. tRNA thiolation is not affected. Extended N-terminus.