Components and assembly of the ESCRT-dependent pathway in fungi
The ESCRT-0 heterodimer is comprised of two subunits, Vps27 and Hse1
(Fig 2), found in S . cerevisiae (Williams and Urbé, 2007).
They are orthologous to the human protein Hrs (h epatocyte growth
factor r egulated tyrosine kinase s ubstrate) and STAM
(s ignal t ransducing a daptor m olecule),
respectively (Raiborg and Stenmark, 2009; Henne et al., 2011; Mosesso et
al., 2019; Xie et al., 2019a). The ubiquitin-interacting motifs (UIMs)
of both subunits comprise of an N -terminal VHS (named after the
proteins V ps27, H rs and S TAM) domain (Ren and
Hurley, 2010). Although both subunits display some structural
resemblance, the Vps27/Hrs structure is slightly different in that it
encompasses a FYVE (named after the proteins F ab1, Y OTB,V ac1 and E EA1) zinc finger domain (Katzmann et al., 2003;
Henne et al., 2011). The cysteine-rich FYVE binds to an endosomal lipid,
phosphatidylinositol (Ptdlns) 3-phosphate (PI3P), that is crucial for
MVB formation and endocytic trafficking and required for the Vps27/Hse1
complex functions and localization (Gillooly et al., 2001; Raiborg et
al., 2003; Katzmann et al., 2003; Xie et al., 2016). InDrosophila melanogaster , inhibition of PI3P alters the
recruitment of Hrs and consequently obstruct MVB formation (Lloyd et
al., 2002; Raiborg et al., 2003). Moreover, defects in the Vps27 UIM do
not impact vesicle budding and delivery onto the MVB lumen, but hinder
sorting of ubiquitinated cargo protein into MVBs (Katzmann et al., 2001,
2002; Piper and Katzmann, 2007; Conibear, 2010).
The ESCRT-I complex is comprised of four subunits, namely Mvb12, Vps23,
Vps37 and Vps28 (Fig 2). However, only Vps23 and Vps28 (Table 1) are
present in F. graminearum and interact with each other (Xie et
al., 2019a). Vps23 and Vps28 also interact with both ESCRT-0 and
ESCRT-II at opposite ends of the complexes (Katzmann et al., 2001; Chu
et al., 2006; Curtiss et al., 2007; Henne et al., 2011). In mammals,
there are multiple isoforms of these ESCRT-I subunits, which may mirror
their variations based on tissue-specificity (Henne et al., 2011). TheN -terminal ubiquitin E2 variant (UEV) domain of the Vps23 subunit
propels to the ESCRT-I stalk, while the UEV binds to the PTAP-like
motifs (P ro-T hr/Ser-A la-P ro) of Vps27, an
upstream MVB sorting component, then recruits the ESCRT complex protein
to the endosomal membrane (Katzmann et al., 2003; Xie et al., 2019a).
The ESCRT-II protein sub-complex is comprised of three subunits, namely
Vps22, Vps36, and Vps25 (Babst et al., 2002b). Through the interaction
between Vps25 and Vps20, ESCRT-II recruits downstream ESCRT-III, thus,
in turn activating this protein sub-complex (Xie et al., 2019a). The
interaction of ESCRT-II with ESCRT-I involves provision of the endosomal
localization, the GLUE (G RAM-l ike u biquitin-binding
in E ap45) domain of Vps36 and Vps28 subunits that bind to
ubiquitin PI3P, while Vps25 binds Vps20 and attach to ESCRT-III, thus
exhibiting the crucial role of ESCRT-II in activating the formation of
ESCRT-III complex (Teo et al., 2006; Hanson et al., 2009; Henne et al.,
2011).
The ESCRT-III sub-complex is comprised of four major subunits, Vps20,
Snf7/Vps32, Vps24, and Vps2 (Fig 2). They occur as monomers in the
cytosol and cluster rapidly on the endosomal membrane into an active
complex (Babst et al., 2002a; Xie et al., 2019b). SnF7 homo-oligomerise
post interaction with Vps20 and it is also the most abundant component
of ESCRT-III and can self-interact as reported in previous studies (Lin
et al., 2005; Teis et al., 2008; Xie et al., 2019b). The major subunits
may contain a few accessory or adaptor proteins such as Did2, Bro1, and
Vps60 (Table 1, Ahmed et al., 2019). ESCRT-III protein sub-complex is
unique from the first three ESCRT complexes because it does not form
part of the stable cytoplasmic complex but rather exist in a closed
autoinhibited condition in the cytoplasm (Henne et al., 2011; Xie et
al., 2019b).
According to Teo et al., (2004), ESCRT-III is triggered by the
interaction between Vps25 and Vps20 from which it forms a complex, as it
is recruited to the endosome. Prior to the activation of ESCRT-III,
ESCRT-I and ESCRT-II complexes link through the interactions between
Vps23 and Vps22, Vps23, and Vps36, and finally Vps28 and Vps36, during
which the subunits Vps25 and Vps36 of ESCRT-II complex directly interact
with ESRCT-III protein complex (Xie et al., 2019b). Despite the
differences in the components of some ESCRT complexes, the sequential
recruitment and clustering of the ESCRT complexes from interactions
among ESCRT protein subunits in F. graminearum has exhibited some
consistencies comparable to the yeast and mammalian models (Xie et al.,
2019b). This suggests that the main interactions of this pathway are
well-conserved in eukaryotes (Martin-serrano et al., 2003; Von Schwedler
et al., 2003; Bowers et al., 2004; Teis et al., 2008; Xie et al.,
2019b).
Lastly, ESCRT-IV disassembles the ESCRT-III complex when ESCRT-II
interacts with Vps20, which recruits SnF7 (Tang et al., 2016). SnF7 has
been found to play roles that are vital to the pathogenic process in
fungi including cell wall integrity, endocytosis, vesicle trafficking,
as well as growth and differentiation (Table 1) (Cheng et al., 2018).
SnF7 then recruits Vps24 and Vps2, thus completing the assembly of
ESCRT-III, shortly after Vps2 engages Vps4 (Obita et al., 2007; Teis et
al., 2008; Henne at al., 2011). For stabilization, SnF7 may also recruit
adaptor proteins from ESCRT-III i.e., Bro1 and Doa4 DUBs (Odorizzi et
al., 2003; Luhtala and Odorizzi, 2004).
For ESCRT-III to disassemble from the membrane, energy is required and
that is usually provided by class I AAA (A TPase a ssociated
with various cellular a ctivities) ATPase Vps4 (Fig 1) (Babst et
al., 1998). As with SnF7, other ESCRT-III accessory proteins also
regulate the interaction between ESCRT-III and Vps4 (i.e., Did2, Vta1 or
Vps60) to modulate the functions of Vps4 in several ways, such as
facilitating the self-interaction of Vps4, Vps4 interaction with
ESCRT-III subunits, or the stimulation of ATP hydrolysis (Ahmed et al.,
2019). Furthermore, Vta1 directly interacts with Vps60 of the ESCRT-III
complex. A super complex that is stabilized by a flexible Vta1 cap is
formed when Vta1 stimulates the ATPase activity of Vps4 (Ahmed et al.,
2019).