The Use of Liver Transglutaminase for Protein Labeling

Guinea‐pig transglutaminase catalyzes the formation of an amide bond between the γ‐carboxyl group of peptide‐bound glutamine and a variety of amines, including fluorescent or spin‐labeled derivatives. A systematic study was conducted with this enzyme in order to establish its usefulness as a protein‐labeling technique. Transglutaminase‐catalyzed amine incorporation was tested with isolated proteins, intact cells, membrane preparations, and virus particles. It was found that, due to its large molecular weight and its specificity, the transglutaminase reaction is limited to highly exposed glutamine residues. For example, proteins such as tubulin and α₂‐macroglobulin were modified while immunoglobulins (IgG) from a variety of sources were not, despite their high glutamine content. The peptide hormone glucagon was modified by transglutaminase, which resulted in the loss of hormonal activity. Most of the intact cells were not labeled with transglutaminase. Only in the case of human fibroblasts the labeling of a few heavy‐molecular‐weight proteins, including large external transformation‐sensitive (LETS) protein, occurred. However, isolated membrane preparations, such as erythrocyte ghosts, sarcoplamic reticulum and retinal disc membranes, were significantly labeled. Intact influenza virions were susceptible to the transglutaminase‐catalyzed reaction; however, only hemaglutinin but not neuraminidase was labeled. The transglutaminase reaction was found useful when specific and limited labeling is required. This technique should be particularly amenable for the introduction of fluorescent probes into membrane proteins and for elucidation of the structure‐function relationship of the proteins.