Class E compartments form in response to ESCRT dysfunction in yeast due to hyperactivity of the Vps21 Rab GTPase.
The endosomal sorting complexes required for transport (ESCRTs) mediate the budding of intralumenal vesicles (ILVs) at late endosomes. ESCRT dysfunction causes drastic changes in endosome morphology, which are manifested in Saccharomyces cerevisiae by the formation of aberrant endosomes known as class E compartments. Except for the absence of ILVs, the mechanistic basis for class E compartment biogenesis is unknown. We used electron microscopy to examine endosomal morphology in response to transient ESCRT inactivation and recovery in yeast expressing the temperature-sensitive mutant vps4(ts) allele. Our results show class E compartments accumulate fourfold the amount of membrane normally present at multivesicular bodies and that multivesicular bodies can form directly from class E compartments upon recovery of ESCRT function. We found class E compartment formation requires Vps21, which is orthologous to the Rab5A GTPase in metazoans that promotes fusion of endocytic vesicles with early endosomes and homotypic fusion of early endosomes with one another. We also determined that class E compartments accumulate GTP-bound Vps21 and its effector, the class C core vacuole/endosome tethering (CORVET). Ypt7, the yeast ortholog of Rab7 that in metazoans promotes fusion of late endosomes with lysosomes, also accumulates at class E compartments but without its effector, the homotypic fusion and protein sorting (HOPS), signifying that Ypt7 at class E compartments is dysfunctional. These results suggest that failure to complete Rab5-Rab7 conversion is a consequence of ESCRT dysfunction, which results in Vps21 hyperactivity that drives the class E compartment morphology. Indeed, genetic disruption of Rab conversion without ESCRT dysfunction autonomously drives the class E compartment morphology without blocking ILV budding.
Department of Molecular, Cellular and Developmental Biology, University of Colorado at Boulder, Campus Box 347, Boulder, CO 80309,  USA.
Matthew R G
MR
Russell
Tess
T
Shideler
Daniel P
DP
Nickerson
Matt
M
West
Greg
G
Odorizzi
D000251:Adenosine Triphosphatases / Q000378:metabolism; D056827:Endosomal Sorting Complexes Required for Transport / Q000378:metabolism / Q000502:physiology*; D011992:Endosomes / Q000201:enzymology / Q000648:ultrastructure*; D007425:Intracellular Membranes / Q000201:enzymology / Q000378:metabolism; D008856:Microscopy, Fluorescence; D012441:Saccharomyces cerevisiae / Q000201:enzymology* / Q000648:ultrastructure; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism*; D020691:rab GTP-Binding Proteins / Q000378:metabolism*
D056827:Endosomal Sorting Complexes Required for Transport; D029701:Saccharomyces cerevisiae Proteins; C107528:VPS4 protein, S cerevisiae; D000251:Adenosine Triphosphatases; C086866:VPS21 protein, S cerevisiae; C079005:YPT7 protein, S cerevisiae; D020691:rab GTP-Binding Proteins