Journal of Experimental Medicine, Vol 153, 1684-1689, Copyright © 1981 by Rockefeller University Press
Biogenesis of membrane-bound and secreted immunoglobulins. II. Two forms of the human alpha chain translated in vitro and processed in vivo as distinct polypeptide chains
JM McCune, SM Fu, G Blobel and HG Kunkel
Structural differences between alpha m (ther heavy chain of membrane IgA)
and alpha s (the heavy chain of secretory IgA) were investigated. Messenger
RNA from the human B 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
alpha chain, that differed in molecular weight. In vivo pulse and
pulse-chase experiments demonstrated that two early biosynthetic forms of
the alpha chain were subsequently modified to yield three intracellular
forms. As shown by endo-beta-N- acetylglucosaminidase H (endo H) treatment,
these forms represent two alpha polypeptide chains, with varying
compositions of N-linked oligosaccharides. Of the two forms of the alpha
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 mu and delta
chains is discussed. These results indicate that alpha m is distinguished
from an alpha 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 (micrometer) and heavy chain of
secretory IgM (microseconds), may reflect a principle common to the
secretory and membrane forms of all immunoglobulin heavy chain classes.
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