Sorting of ubiquitinated-protein cargo into ILVs
Ubiquitination plays major roles in trafficking plasma membrane proteins
into the endosomal system (Schwihla and Korbei, 2020). Endosomal
ubiquitin modification is critical for ESCRT recognition and sorting of
ubiquitin-modified protein cargo into ILVs (Williams and Urbé, 2007;
Villarroya-Beltri et al., 2014). This endosomal modification facilitates
ubiquitinated proteins to lysosomes through MVBs (Williams and Urbé,
2007). Ubiquitination also serves as essential signal for proteins
lacking signal peptides recruited to the ESCRT-dependent pathway during
MVB sorting (Piper and Luzio, 2001; Agrawal et al., 2010)
Early endosomes, which originate from the trans -Golgi network
(TGN) as part of the endocytic membrane transport pathway, are major
sorting stations that accept molecules from the extracellular
environment. On the other hand, MVBs are prelysosomal organelles acting
during endocytosis to regulate incoming and outgoing traffic through
homotypic and heterotypic fusion events (Luzio et al., 2003; Luzio et
al., 2007; Huotari and Helenius, 2011). Although, homotypic fusion of
MVBs occurs with lysosomes and autophagosomes, heterotypic fusion is
different in that it can occur with the plasma membrane. But in both
cases, fusion results in the delivery of endocytosed cargo. In the case
where MVBs fuse with the plasma membrane, ILVs are released into the
extracellular space as exosomes, and this has been implicated in
cell-cell and trans -kingdom communication including plant-fungal
interaction (Cai et al., 2018).
The sorting of MVB is important for a series of biological events,
including endosomal sorting, organelle biogenesis, and vesicular
transportation (Colombo et al., 2014; Yáñez-Mó et al., 2015; Hyka et
al., 2017). Assembly of these molecular cargos takes place inside
membrane microdomains of the MVBs and on the plasma membrane (Kuipers et
al., 2018). Several ESCRT proteins have been implicated as constituents
enriched in EVs, particularly 30-150 nm cup-shaped vesicles (i.e.,
Tsg101, a protein associated with ESCRT-I, and Alix, an accessory
protein associated with ESRCT-III, both of which make up components of
the EVs) and are frequently used as biomarkers for them (Juan and
Fürthauer, 2018; Anand et al., 2019).
Prior to being sorted into ILVs, protein cargo transverse between
membrane-bound protein complexes that recognize and bind to the
ubiquitin chains attached to them (Paez Valencia et al., 2016). Equipped
with the ubiquitin-interacting domains essential for cargo sorting, the
ESCRT complexes -0, -I and -II interact stringently with ubiquitinated
cargos and traffic them to the endosomal MVB pathway (Fig 3(B) ). These sub-complexes can recognize and retain endosomal
ubiquitinated cargos, and transport them into MVBs (Anand et al., 2019;
Mossesso et al., 2019). In addition, endocytosis modulators such as
epsins (e.g., Epsin15) possess ubiquitin-interacting motifs (UIM) that
function in ubiquitin recognition (Mayers and Audhya, 2012). The yeast
homologs of epsin, Ent1 and Ent2, are reportedly essential for
endocytosis of ubiquitin-dependent α-factor receptor that requires
ESCRT-0 Vps27 for sorting (Shih et al., 2002; Raiborg et al., 2003).
Finally, membrane scission of the ECRT complex is promoted by the
recruitment of the ESCRT-III assembly together with its accessory
proteins including Bro1 or Vps60 (Moreno-Gonzalo et al, 2014;
Stoorvogel, 2015; Fig 2).