Characterization of the mRNA for mouse muscle acetylcholine receptor alpha subunit by quantitative translation in vitro.

The nicotinic acetylcholine receptor of mammalian skeletal muscle is a multisubunit membrane glycoprotein whose synthesis is regulated by developmental and physiological cues. We report here the identification and characterization of the primary translation product of alpha subunit mRNA. The alpha subunit synthesized in rabbit reticulocyte lysate is approximately 2000 larger in apparent molecular weight than the native alpha subunit polypeptide found in acetylcholine receptor. Evidence from peptide maps and the effect of co-translational incubation with dog pancreas microsomes suggests that the in vitro product differs in two ways from native alpha subunit: 1) it is synthesized with an NH2-terminal signal peptide which is removed in vivo, and 2) the in vitro product is not glycosylated. We have characterized the alpha subunit mRNA activity by using a quantitative the membrane-bound polysome fraction. It is poly(A+) and approximately 2000 nucleotides long. Finally, we have shown that in BC3H-1 cells, alpha subunit mRNA is regulated developmentally. We detected a 10-fold increase in the relative abundance of alpha subunit mRNA in cells which had undergone the transition from log phase growth to differentiated myoblast.