Association of newly synthesized tubulin with brain microsomal membranes

Tubulin has been found to be synthesized on both membrane-bound and free polyribosomes prepared from brain. Cell-free studies indicate that tubulin made on rough microsomes is incorporated into the endoplasmic reticulum membrane as it is synthesized. This tubulin remains associated with the membrane after sedimentation and washing. The tubulin is not removed from the membrane after stripping ribosomes from the membranes in KCl-puromycin, followed by repeated washing by either sedimentation or flotation in 0.05 M-KCl. The membrane tubulin is partially susceptible to proteolysis by trypsin and chymotrypsin: beta-tubulin is more accessible to the proteases than in alpha-tubulin. Nonionic detergents extract mostly beta-tubulin from the microsomal membrane. Newly synthesized tubulin which has been extracted from microsomal membranes in 0.5% Nonidet P-40, coassembles and disassembles with carrier microtubule protein. The insertion of newly synthesized tubulin into endoplasmic reticulum membrane may be the first step in the incorporation of tubulin into the plasma membrane.