The glycoprotein nature and biosynthetic relationship of two molecularly distinct species of primate T-cell growth factor.

T-cell growth factor (TCGF) produced by the MLA 144 gibbon ape T-lymphosarcoma cell line was biosynthetically radiolabelled with [35S]methionine and isolated by reverse-phase h.p.l.c. [Milstone & Parker (1983) Biochem. Biophys. Res. Commun. 115, 762-768]. Two predominant species, of Mr 16300 and pI 6.8 and of Mr 14300 and pI 7.5, were resolved. Analysis of TCGF labelled with a mixture of [3H]glucosamine and [14C]methionine demonstrated that only the 16300-Mr protein contained detectable carbohydrate label, approx. 50% of which was present in sialic acid residues, which were largely responsible for the charge difference observed between the two forms of TCGF. The 14300-Mr protein was labelled within 5 min after addition of [35S]methionine and was the predominant intracellular form of TCGF, whereas the 16300-Mr protein was not detected until 25 min later and was the predominant extracellular form of TCGF. Pulse-chase experiments were consistent with the hypothesis that the 14300-Mr protein is an intracellular precursor that is glycosylated to form the 16300-Mr protein, which is then preferentially secreted from the cell.