The Journal of Cell Biology, Vol 88, 37-41, Copyright © 1981 by The Rockefeller University Press
Membrane-bound ribosomes of myeloma cells. V. Subcellular distribution of immunoglobulin mRNA molecules
B Mechler
The subcellular distribution of the most abundant mRNA sequences,
particularly those of the immunoglobulin heavy (Ig H) and light (IG L)
chain mRNA sequences, of MOPC 21 (P3K) mouse myeloma cells has been
examined by translating the mRNA of various subcellular fractions in a
messenger-dependent reticulocyte lysate (MDL) and by identifying Ig
products with the use of a specific antiserum. Analyses of the distribution
of the mRNA template activity and the translation products by SDS
polyacrylamide gel electrophoresis reveal that approximately 85% of the
mRNA present in the free ribosomal fraction is incorporated into polysomes
and that the remainder is present as mRNP particles. On the endoplasmic
reticulum (ER) the mRNA is found entirely in polysomes. In general, the
size class of free (F) and membrane-bound (MB) polysomes corresponds to the
size of their translation products. Thus, mRNAs coding Ig H (5.0 x 10(5)
daltons in size) and Ig L (2.5 x 10(5) daltons in size) are incorporated
into polysomes formed of 12 and 6 ribosomes, respectively. About 10% of the
Ig mRNAs are not bound to membranes. A third of these are associated with
mRNPs and the remainder incorporated into F polysomes of the same size as
the Ig-synthesizing MB polysomes.
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