Tubulin and tubulin-colchicine complex bind to brain microsomal membrane in vitro

Brain tubulin was labeled in vitro by post-translational incorporation of [14C]-tyrosine or in vivo by intra-cranial injection of [3H]-leucine. The labeled protein was purified by ion-exchange chromatography. After incubating at 37 degrees C with a microsomal membrane preparation from rat brain, part of the labeled soluble tubulin became sedimentable at high-speed centrifugation. This was independent of the native configuration of tubulin, the state of tyrosination of the COOH-terminus, or the presence of 100 microM colchicine in the mixture. In addition, the double-labeled tubulin-colchicine complex obtained from the binding of [3H]-colchicine to [14C]-tyrosinated tubulin, bound to the membrane preparation to the same extent as [14C]-tyrosinated tubulin. The data show that either tubulin or the complex resulting from its binding to colchicine distributed between the soluble and the membrane fractions when mixed at 37 degrees C with a microsome preparation. Seemingly, the site for colchicine binding to tubulin needs not to be free for the protein-membrane association.