The T3 complex on human thymus-derived lymphocytes contains two different subunits of 20 kDa.

The human cell surface antigen T3 is involved in several T lymphocyte specific functions, as determined by the effect of monoclonal antibodies (OKT3, anti-Leu-4, UCHT1) directed at this molecular structure. The main target antigen of these monoclonal antibodies is a glycoprotein of 20 kDa. It is associated with four, less predominant, structurally distinct glycoproteins of 25-28 kDa, 37 kDa and 44 kDa. Of these molecules only the 20-kDa T3 antigen could be labeled with the hydrophobic reagent 5-iodonaphthyl-1-azide (INA). Here we present evidence that the main 20-kDa T3 antigen is comprised of, in fact, two structurally different molecules. One of these is a glycoprotein with a protein backbone of 14 kDa, the other is an unglycosylated protein of 20 kDa. This unglycosylated protein is labeled specifically with INA. Additional evidence for the existence of two different 20-kDa T3 antigens is provided by studies using the enzymes endo-beta-N-acetylglucosaminidase H and endo-beta-N-acetylglucosaminidase F and the drug tunicamycin. We hypothesize that the specific susceptibility to labeling with INA of the unglycosylated 20-kDa T3 form reflects a positioning in the lipid bilayer different from that of the glycosylated 20-kDa T3 form.