Membrane insertion of gap junction connexins: polytopic channel forming membrane proteins

The translation of GJ proteins in the presence of canine pancreatic microsomes resulted in the generation of specific translation products that were '~2-2.5 kD smaller in size than the full-size proteins generated in the absence of microsomes, based on the electrophoretic mobility shift on the protein gels (marked as a' and/~' GJ protein in the figures). The faster migrating translation products were generated with all connexin cRNAs translated in these assays (Fig. 1, lanes 3 -10, 13, 14, 17, 18, 20, and 22) . This finding was surprising because no general connexin modification has been described that could account for the observed increase in electrophoretic mobility. Endogenous ~1 GJ proteins isolated from mammalian tissues correspond in their electrophoretic mobility to the full-size translation products (compare Fig. 3 A) . Translocation reactions incubated for very short time periods always showed equivalent amounts of modified and unmodified translation products in the individual reactions, even at time periods when the translation of connexin polypeptides was still continuing (Fig. 1, lanes 3-8; shown for c~ GJ protein only). This result indicates that the processing occurred cotranslationally or, at least, in close relationship to the translocation reaction.