Regulation of intracellular trafficking of the EGF receptor by Rab5 in the absence of phosphatidylinositol 3-kinase activity

Rab5 and phosphatidylinositol S-kinase (PI3K) have been proposed to co-regulate receptor endocytosis by controlling early endosome fusion. However, in this report we demonstrate that inhibition of epidermal growth factor (EGF)-stimulated Pl3K activity by expression of the kinase-deficient Pl3K p110 subunit (p110 Delta kin) does not block the lysosomal targeting and degradation of the EGF receptor (EGFR). Moreover, inhibition of total Pl3K activity by wortmannin or LY294002 significantly enlarges ECFR-containing endosomes and dissociates the early-endosomal autoantigen EEA1 from membrane fractions. However, this does not block the lysosomal targeting and degradation of EGFR. In contrast, transfection of cells with mutant Rab5 S34N or microinjection of anti-Rabaptin5 antibodies inhibits EGFR endocytosis. Our results, therefore, demonstrate that Pl3K is not universally required for the regulation of receptor intracellular trafficking. The present work suggests that the intracellular trafficking of EGFR is controlled by a novel endosome fusion pathway that is regulated by Rab5 in the absence of Pl3K, rather than by the previously defined endosome fusion pathway that is co-regulated by Rab5 and Pl3K.