Removal of the projections from cytoplasmic microtubules in vitro by digestion with trypsin.

Trypsin was found to selectively destroy the high molecular weight (HMW) proteins associated with microtubules. Both major groups of high molecular weight bands observed by sodium dodecylsulfate-polyacrylamide gel electrophoresis were destroyed at the same rate (t 1/2 = 1 min), while tubulin was little affected under the conditions employed. Destruction of the high molecular weight bands was correlated with the disappearance of the lateral projections observed on the surface of microtubules. The trypsin-treated protein was capable of reassembly to form microtubules and showed characteristic ring-shaped structures at low temperature. Analysis of fragments formed from the high molecular weight proteins indicated the initial appearance of a fragment of M = 255,000 which failed to bind to microtubules. These results suggest that a small fragment of the high molecular weight protein molecule may remain bound to the microtubule after removal of the lateral projection, and that this fragment may retain the ability to promote microtubule assembly.