The Ccz1-Mon1 Protein Complex Is Required for the Late Step of Multiple Vacuole Delivery Pathways

Abstract

Mon1 and Ccz1 were identified from a gene deletion library as mutants defective in the vacuolar import of aminopeptidase I (Ape1) via the cytoplasm to vacuole targeting (Cvt) pathway. The mon1⌬ and ccz1⌬ strains also displayed defects in autophagy and pexophagy, degradative pathways that share protein machinery and mechanistic features with the biosynthetic Cvt pathway. Further analyses indicated that Mon1, like Ccz1, was required in nearly all membrane-trafficking pathways where the vacuole represented the terminal acceptor compartment. Accordingly, both deletion strains had kinetic defects in the biosynthetic delivery of resident vacuolar hydrolases through the CPY, ALP, and MVB pathways. Biochemical and microscopy studies suggested that Mon1 and Ccz1 functioned after transport vesicle formation but before (or at) the fusion step with the vacuole. Thus, ccz1⌬ and mon1⌬ are the Compartmentalization allows eukaryotic cells to regulate intracellular functions by separating competing reactions and localizing enzymes and substrates at specific locations within the cell. Efficient compartmentalization necessitates dynamic protein trafficking processes by which cells are able to establish and maintain the identity and function of each organelle. The vacuole (lysosome) of the yeast Saccharomyces cerevisiae plays a central role in the turnover of cytoplasmic organelles, degradation of intracellular/extracellular components, and maintenance of cellular physiology (1). To carry out these functions, the vacuole maintains a variety of degradative enzymes. Both resident hydrolases and their substrates arrive at this destination through a variety of sorting pathways. The main routes by which vacuolar hydrolases are delivered to this organelle are the carboxypeptidase Y (CPY), 1 alkaline phosphatase (ALP), and multivesicular body (MVB) pathways, which involve transit through a portion of the secretory pathway, and the cytoplasm to vacuole targeting (Cvt) pathway by which the cargo molecules are packaged as cytosolic membrane-bound intermediates (2, 3). Resident proteins are also transmitted by inheritance from mother cell vacuoles to daughter cells during cell division (4). Substrates enter the vacuole through endocytosis, autophagy and the vacuole import and degradation pathway (reviewed in Ref. 5). One common feature in all of these processes is membrane fusion. The membrane fusion mechanism acts to ensure specificity for the directed movement of proteins while also maintaining the distinct composition of each organelle within the highly compartmentalized eukaryotic cell.The cytoplasm to vacuole targeting pathway that is used to deliver the soluble hydrolase aminopeptidase I (Ape1) to the vacuole has been under investigation (for reviews see Refs. 2, 5, and 6). Under vegetative conditions, precursor Ape1 (prApe1) is assembled into a large Cvt complex composed in part of multiple prApe1 dodecamers in the cytosol that becomes enwrapped within a double-membrane Cvt vesicle (7). Upon completion, the cytosolic Cvt vesicle target...

Autophagic Processes in Yeast: Mechanism, Machinery and Regulation

“…This fusion step involves components that are common to other transport processes that terminate at the vacuole. Thus, the Rab protein Ypt7, its GDP exchange factor complex Ccz1-Mon1, and SNARE proteins including Vam3, Vam7, Vti1, and Ykt6, along with the class C Vps/HOPS complex, are needed for tethering and fusion (Darsow et al 1997;Rieder and Emr 1997;Ishihara et al 2001;Meiling-Wesse et al 2002;Wang et al 2002;Nair et al 2011;Polupanov et al 2011). The fusion of the autophagosome outer membrane with the vacuole-limiting membrane releases the inner vesicle into the lumen (Figure 1).…”

Autophagy: molecular machinery for self-eating

“…81 Ccz1 and Mon1 form a complex and also function in homotypic vacuole fusion. 82 The Ccz1-Mon1 complex form part of a common fusion machinery consisting of the SNARE proteins Vam3, Vti1 and Vam7, the NSF Sec18, the a-SNAP Sec17, the Rab GTPase Ypt7 and the class C Vps/HOPS complex.…”

The late stages of autophagy: how does the end begin?

“…27 The Ccz1-Mon1 protein complex acts together with Ypt7 in these fusion processes. [37][38][39] The Ypt7 homologue, Rab7, has been shown to be involved in autophagosome-lysosome fusion in mammalian cells, 31,40,41 Vam3, Vam7 and Vti1 are SNARE proteins involved in fusion in yeast. 42,43 The key question remaining to be answered is what is the SNARE molecule residing in autophagosomes (corresponding to v-SNARE) and how it is delivered.…”