Vacuolar protein sorting genes (Vps) facilitate the assembly of vacuoles
Fungal vacuoles are complex cellular organelles involved in several functions including homeostatic regulation and degradative processes among others (Klionsky et al. 1990). During EV biogenesis, fusion of MVBs occurs with vacuoles and autophagosomes and the MVB-endocytosed cargo is delivered and then undergo degradation as means of cellular homeostasis (Fig 1 (D) , Luzio et al., 2003; Luzio et al., 2007). Induced autophagy has been shown to result in reduced EV release due to increased fusion of the MVBs to the autophagosomes (Fader et al., 2008). Though little is known about directive mechanisms involved in trafficking of MVBs to either the plasma membrane or the vacuole, these organelles serve an important role in maintaining cytosolic balance.
Vacuolar proteins are recognized by certain cellular components and are sorted, packaged into transport vesicles, and delivered to the vacuole through a prevacuolar compartment (Hedman et al., 2007). This process involves the assembly of the Vps genes (e.g., Vps 23 or Vps 27) into the five sub-complexes of the pathway, which in a stepwise ‘modus operandi , interact, recognize and sort ubiquitinated cargo (Fig 2, Mosesso et al., 2019). These proteins are encoded by class E vacuolar protein sorting genes that are structurally classified as type A-F. Among these, the first Vps genes were classified into type A, B and C, with class D, E, and F added later (Hedman et al., 2007). These Vps genes are associated with the formation of the vacuole. For instance, mutants of the class A genes exhibit defects in acidification of the vacuole, class B mutants exhibit small-vacuole like organelles, which together with class F mutants are characterized by large to fragmented small vacuole-like structures (Banta et al., 1988; Hedman et al., 2007).
Class C Vps is associated with normal assembly of the vacuole as mutants display severely aberrant vacuole formation and sensitivity to osmotic stress in S. cerevisiae (Banta et al., 1988). Although their absence has minimal effects in vacuolar biogenesis, and trafficking of endocytosed proteins, class D gene products are limited to the vesicular pathway (Bryant et al., 1998). Class E Vps are important in forming unique class E compartments that house the vacuolar hydrolase carboxypeptidase S (CPS), a transmembrane protein required in the vacuole (Raymond et al., 1992; Shaw et al., 2001; Coonrod et al., 2010). While CPS accumulates in class E compartments, CPS that fails to reach the vacuole lumen remains in the limited membranes of the vacuole (Reggiori et al., 2001; Bowers et al., 2005; Piper et al., 2007). In yeast, about 15 components of the class E Vps family, most of which associated with MVB sorting and augmented endosomal compartments, were reported (Katzmann et al., 2001).
It has also been demonstrated in yeasts and mammals that Vps proteins interact sequentially. This results in the development of various cellular events such as scission, transport, and the formation of distinct budding membranes (Hurley and Hanson, 2010; Henne et al., 2011; Schuh and Audhya, 2014; Tang et al., 2016). Hence, ESCRT-I and ESCRT-II have been identified as crucial elements in trafficking ubiquitinated cargo to the vacuole. ESCRT- 0 on the other hand, is not as essential in either mammals or yeast for the aforementioned functions, which is in accordance with the dynamic endosomal pathway in plants where ESCRT- 0 is also absent (Henne et al., 2011; Richardson et al., 2011; Hurley, 2015).