Novel mechanism of action of monoclonal nonspecific suppressor factor (MNSF)

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Monoclonal nonspecific suppressor factor (MNSF), a lymphokine produced by murine T cell hybridoma, possesses pleiotrophic antigen-nonspecific suppressive function. We have shown that 70-kDa MNSF comprises an 8-kDa ubiquitin-like polypeptide (Ubi-L) and 62-kDa TCRα-like molecule. Ubi-L binds specifically to its 82-kDa receptor protein on target cells. In the current study, We have further characterized the biochemical nature of the TCRa-like molecule. The 62-kDa protein was separated into two species of 46-kDa and 16-kDa on reverse-phase HPLC. Anti-TCRα monoclonal antibody recognized the 46-kDa, but not 16-kDa protein. Anti-TCRβ monoclonal antibody failed to recognize these proteins. Ubi-L conjugated to the 46-kDa protein, whereas Ubi-L lacking its C-termiai Gly-Gly did not. Although Ubi-L was labile both to heating at 56℃ and to acidification to pH 4, Ubi-L46-kDa protein complex was unaffected by these treatments. In addition, the 46-kDa protein elongated the Ubi-L-induced protein tyrosine phosphorylations in concanavalin A-activated murine T helper type 2 clone, D10 cells. One of the four tryptic peptide sequences derived from the 46-kDa protein was in alignment with a related sequence found in the Ja regjon of the TCRa, including the highly conserved motif F-G-X-G-T-X-L.