[Effect of nocodazole on endocytosis of epidermal growth factor receptor]

During endocytosis EGF-receptor complexes are transported from early peripheral endosomes to late juxtranuclear-located endosomes to be then degraded in lysosomes. It is suggested that such a spatial organization of endosomal compartments is maintained by microtubule system and is necessary for lysosomal degradation of endocytosed cargo. In the present work, a study was made of the influence of Nocodazole, a microtubule depolymerizing agent, on endocytosis of fluid phase marker HRP and EGF entering the cell via receptor-mediated endocytosis. By subcellular fractionation in Percoll gradient it was shown that Nocodazole did not affect HRP internalization but stimulated its accumulation in a fraction sedimented together with late endosomes, thus preventing HRP delivery to lysosomes. On the contrary, Nocodazole exerted no influence on dynamics of compartmentalization and lysosomal degradation of EGF-receptor complexes. Moreover, no alterations were found in the functioning of a so well-known EGF-stimulated signal transduction pathway as MAP-kinase cascade. At the same time microtubule depolymerization dramatically changed the morphology of endosomal compartments abolishing juxtranuclear localization of late endosomes. Our data suggest that translocation of EGF-receptor complexes is not necessary for their normal lysosomal processing. Rab7, traditionally considered as a marker of late endosomes, has been found to demonstrate in Nocodazole-treated cells, in contrast to the control, a low extent of co-localization with endosomal structures. It could be supposed that the role of Rab7 is not so much to mediate early-to-late endosome transition as to maintain spatial organization of endosomal apparatus by mediating endosome-cytoskeleton interactions.