Biogenesis of membrane-bound and secreted immunoglobulins. II. Two forms of the human α chain translated in vitro and processed in vivo as distinct polypeptide chains

Abstract

Structural differences between α[m] (the heavy chain of membrane IgA) and α[s] (the lymphoblastoid line 32a.1, expressing both membrane and secretory IgA, was translated in a wheat germ cell-free system, resulting in the synthesis of two primary translation products for the α chain, that differed in molecular weight. In vivo pulse and pulse-chase experiments demonstrated that two early biosynthetic forms of the α chain were subsequently modified to yield three intracellular forms. As shown by endo-β-N-acetylglucosaminidase H (endo H) treatment, these forms represent two α polypeptide chains, with varying compositions of N-linked oligosaccharides. Of the two forms of the α chain remaining after endo H treatment, only the form with the lowest molecular weight was associated with cells after long chase periods. The possible significance of this difference from the results with μ and δ chains is discussed. These results indicate that α[m] is distinguished from α[s] by a difference in both primary structure and intracellular processing. The functional consequences of this distinction, previously shown for the heavy chain of membrane IgM (μ[m]) and heavy chain of secretory IgM (μ[s]), may reflect a principle common to the secretory and membrane forms of all immunoglobulin heavy chain classes.