Functions as a protein modifier covalently attached to lysine residues of substrate proteins, as well as a sulfur carrier in tRNA thiolation. The protein modification process is termed sampylation and involves the formation of an isopeptide bond between the SAMP2 C-terminal glycine carboxylate and the epsilon-amino group of lysine residues on target proteins. Is able to form polymeric chains with itself at Lys-58, similar to ubiquitin and other ubiquitin-like proteins. May serve as a proteolytic signal in the cell to target proteins for degradation by proteasomes