In vitro synthesis of the major lens membrane protein.

The biosynthetic activity of a polyribosomal fraction isolated from the lens fiber plasma membrane-cytoskeleton complex by DNase I treatment has been assayed. After translation of these polyribosomes in a reticulocyte cell-free system and analysis of the products by electrophoresis in sodium dodecyl sulfate gels, the preferential synthesis of a protein with an apparent molecular weight of 26,000 was observed. By means of immunochemical characterization we showed that this protein, which seems not to be synthesized by "free" polyribosomes, is identical with the major intrinsic plasma membrane protein MP26 of lens fibers. Upon storage, the molecular weight of the newly synthesized protein decreases to about 22,000, a phenomenon that has previously been observed for MP26 in isolated plasma membranes and that may be caused by the presence of a specific proteolytic cleaving site in the protein.