Association of Newly Synthesized Tubulin with Brain Microsomal Membranes

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Abstract
Tubulin was synthesized on membrane-bound and free polyribosomes prepared from pig brain. In cell-free preparations tubulin made on rough microsomes was incorporated into the endoplasmic reticulum membrane as it was synthesized. This tubulin remained associated with the membrane after sedimentation and washing. The tubulin was not removed from the membrane after stripping ribosomes from the membranes in KCl-puromycin, followed by repeated washing by sedimentation or flotation in 0.5 M-KCl. The membrane tubulin was partially susceptible to proteolysis by trypsin and chymotrypsin; .beta.-tubulin was more accessible to the proteases than .alpha.-tubulin. Nonionic detergents extracted mostly .beta.-tubulin from the microsomal membrane. Newly synthesized tubulin extracted from microsomal membranes in 0.5% Nonidet P-40, coassembled and disassembled with carrier microtubule protein. The insertion of newly synthesized tubulin into endoplasmic reticulum membrane may be the 1st step in the incorporation of tubulin into the plasma membrane.