CHARACTERIZATION OF A THIOREDOXIN-RELATED SURFACE PROTEIN

A surface-associated sulphydryl (thiol) protein (SASP) constitutively present in most nucleated cells was purified from human THP-I monocyte s and rat C6 glioma cells. The human protein was similar in mass and i soelectric point and had the same N-terminal amino acid sequence to ad ult T-cell leukaemia-derived factor (ADF), a growth factor secreted by human lymphoid cells which is able to induce increased expression of interleukin-2 receptors. A further internal amino acid sequence, deter mined following cleavage of human SASP with cyanogen bromide, was also identical to the corresponding sequence deduced for ADF. Samples of S ASP were able to reductively depolymerize human immunoglobulin, a prop erty shared with thioredoxin, a ubiquitous protein, almost identical t o ADF, with an essential function in many thiol-dependent reducing rea ctions. Furthermore, SASP purified from rat C6 glioma cells had an ide ntical N-terminal amino acid sequence to that deduced for rat liver th ioredoxin, showing that they were both members of the same family of p roteins. The use of membrane-impermeable thiol reagents indicated that SASP was predominantly a cell-surface protein, and was not normally s ecreted. This SASP protein appeared to be a surface-associated form of thioredoxin that was constitutively present in a wide range of cells and was related to ADF, a secreted form of the same protein.