Can coated vesicles bind directly to microtubules?

Using an ultrathin-sectioning electron microscopic procedure, no efficient binding of coated vesicles to microtubules (both purified from brain tissue) could be achieved, independently of the presence of microtubule-associated proteins. Addition of the ATP analogue AMP-PNP or the inorganic tripolyphosphate, known to cause tight associations of (uncoated) vesicles to microtubules by means of specific motor proteins, could not enhance the binding efficiency. Moreover, crude preparations of clathrin, the major protein of the coat, did not affect the turbidity course of microtubule assembly. These results were confirmed by electrophoresis indicating that within the preparations of microtubule protein, obtained by temperature-dependent cycles of disassembly/reassembly, constituents of coated vesicles were not present. Beside this, coated vesicles have never been found among microtubules reconstituted in vitro. Vice versa, preparations of coated vesicles were completely free of microtubules. Our results suggest that further proteins should be involved in the binding of coated vesicles to microtubules, if there is indeed a biologically relevant interaction.