Membrane-bound ribosomes of myeloma cells. VI. Initiation of immunoglobulin mRNA translation occurs on free ribosomes.

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Abstract
Ig H-chain and Ig L-chain mRNA molecules have been released from the endoplasmic reticulum (ER) membranes as free (F) mRNP particles when MOPC 21 (P3K) mouse myeloma cells are exposed to a hypertonic initiation block (HIB). The subsequent fate of these mRNA sequences has been examined when the cells are returned to normal growth medium. Upon return to isotonicity, all previously translated mRNA molecules reassociate with ribosomes and form functional polysomes. Ig H mRNA is incorporated first into F polysomes and then into membrane-bound (MB) polysomes. Kinetic studies indicate that the time of passage of Ig H mRNA in F polysomes is .apprx. 30 s, during which time a nascent polypeptide chain of .apprx. 80 amino acids would have been completed. When the rate of polypeptide elongation is depressed with emetine during the recovery from HIB, Ig H and L mRNA molecules accumulate in small F polysomes. The formation of Ig-synthesizing polysomes proceeds in the sequence: mRNA .fwdarw. F polysomes .fwdarw. MB polysomes. With the additional observation that during HIB recovery puromycin completely prevents the reassociation of Ig mRNA with the ER, these findings support a model of MB polysome formation in which the specificity of membrane attachment is determined by the nature of the N-terminal amino acid sequence of the nascent polypeptide chain.