[The association of a protein regulator of intracellular Rab7 membrane transport with endosomes containing an epidermal growth factor receptor with activated and inactivated tyrosine kinase].

By double indirect immunofluorescent microscopy, Rab7, traditionally considered as a late endosomal marker, has been demonstrated to colocalize with an internalized epidermal growth factor receptor (EGFR) possessing active and inactive tyrosine kinase (TK). The epidermal growth factor (EGF), which induces TK activity of EGFR, and monoclonal antibody Mab 108, which does not, have been exploited as ligands to stimulate endocytosis of EGFR. Colocalization between EGFR and Rab7 has been detected at both early (10 min) and delayed (60 and 120 min) endocytosis of EGFR, while it turned out to be much more obvious at the later ones. A comparison between EGFR-mediated and peroxidase fluid-phase endocytoses has revealed that Rab7 failed to be recruited on endosomal structures, containing peroxidase, even after 180 min endocytosis. Subcellular fractionation of endosomes containing 125I-EGF and 125I-Mab 108 in Percoll density gradients, in parallel with the analysis of ligand degradation, have verified the efficient transition of EGF-receptor complexes into the late endosomes and retention of Mab 108-receptor complexes within the early (sorting) endosomes. Taken together, the data cotained suggest that endogenous Rab7 is able to be recruited not only on late but also on maturating sorting EGFR-containing endosomes, thus mediating sorting along the EGFR endocytotic pathway.