Rab4 affects both recycling and degradative endosomal trafficking

The small GTPases Rab4, Rab5 and Rab7 are endosomal proteins which play imp ortant roles in the regulation of various stages of endosomal trafficking, Rab4 and Rab5 have both been localized to early endosomes and ha,e been sho wn to control recycling and endosomal fusion, respectively. Rab7, a marker of the late endosomal compartment, is involved in the regulation of the lat e endocytic pathway. Here, we compare the role of Rab4, Rab5 and Rab7 in ea rly and late endosomal trafficking in HeLa cells monitoring ligand uptake, recycling and degradation. Expression of the Rab4 dominant negative mutant (Rab4AS22N) leads to a significant reduction in both recycling and degradat ion while, as expected, Rab7 mutants exclusively affect epidermal growth fa ctor (EGF) and low density lipoprotein degradation. As also expected, expre ssion of the dominant negative Rab5 mutant perturbs internalization kinetic s; and affects both recycling and degradation. Expression of Rab4WT and dom inant positive mutant (Rab4AQ67L) changes dramatically the morphology of th e transferrin compartment leading to the formation of membrane tubulus, The se transferrin positive tubules display swellings (varicosities) some of wh ich are positive for early endosomal antigen-1 and contain EGF, We propose that the Rab4GTPase is important for the function of the early sorting endo somal compartment, affecting trafficking along both recycling and degradati ve pathways. (C) 2001 Federation of European Biochemical Societies. Publish ed by Elsevier Science B.V. All rights reserved.