Vacuoles can fuse with late endosomes (multivesicular body pathway) or possibly with vesicles derived from endosomes (carboxypeptidase Y pathway), with Golgi complex-derived vesicles (alkaline phosphatase pathway), and with themselves (homotypic fusion). In all of these scenarios, cells use an identical fusion machinery, which consists of SNARE proteins, a rab-GTPase, and the class C vps protein complex, also known as the HOPS (homotypic fusion and vacuole protein sorting) complex (18, 67, 94, 101, 137, 139).
It is not surprising that the same components are exploited for the fusion of autophagosomes and Cvt vesicles. Precursor Ape1 maturation was shown to be blocked in strains where the two vacuolar SNARE proteins, Vam7 and Vti1, and the subunits of the HOPS complex, Vps39 (the product of a gene allelic to CVT4) and Vps41 (the product of a gene allelic to CVT8), were nonfunctional (26, 32, 93). These results should be considered with caution because these proteins are essential for protease delivery to the vacuole, and the observed defect in prApe1 processing could be indirect (18, 26, 67, 76, 93). However, the possibility of their direct role has been highlighted because strains with defects in other components of this fusion machinery, such as the vacuolar SNARE protein Vam3, the small rab-GTPase Ypt7, and two other subunits of the HOPS complex, Vps16 (the product of a gene allelic to CVT15) and Vps18, have been shown to accumulate autophagosomes or Cvt vesicles (2, 7, 19, 28, 50, 89, 97).
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