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).