Posttranslational modification of protein cargo via
ubiquitination
Ubiquitin is a highly conserved regulatory protein, consisting of 76
amino acid residues (Wang et al., 2012; Ahmed et al., 2019; Wang et al.,
2019). Ubiquitination is a process in which a ubiquitin is covalently
attached to a targeted lysine residue on the cytoplasmic tail of a
transmembrane protein (Wang et al., 2012; Ahmed et al., 2019).
Monoubiquitination serves as a signal for the lysosomal targeting
process, where monoubiquitinated proteins are trafficked to the lysosome
(Raiborg et al., 2003; Anand et al., 2019). By contrast,
polyubiquitinated proteins are intended for proteolysis, (Hicke et al.,
2001). Ubiquitination thus appears to serve centrally both in
endocytosis and protein turnover (Leung et al., 2008; Mosesso et al.,
2019).
The process of ubiquitination (and de-ubiquitination) takes place along
the endosomal membrane. Ubiquitination involves the activities of three
enzymes, namely ubiquitin-activating enzyme (E1), ubiquitin-conjugating
enzyme (E2), and ubiquitin ligase (E3), which are used for the
development of iso-peptide bonds between the C-terminal glycine of
ubiquitin and the amine of the lysine (K) (e.g., -63 etc.) residue of
the target protein (Bhoj and Chen, 2009; Liu and Xue, 2011; Schwihla and
Korbei, 2020).
The E3 ligase mostly regulates the specificity of the targeted protein
(Oh et al., 2012). So far, only seven lysine residues within the
ubiquitin protein are used in the ubiquitination reaction to create
ubiquitination chains (i.e., K6, -11, -27, -29, -33, -48, and -63) and
may lead to mono-ubiquitinated or poly-ubiquitinated substrates (Wang et
al., 2012; Schwihla and Korbei, 2020). The most common polyubiquitin
chains are, however, K48- and K63-linked polyubiquitin chains, with K63
reportedly associated with endocytic transportation (Rodrigo-Brenni et
al., 2010, Paez Valencia et al., 2016). In yeast, monoubiquitination
alone is sufficient to stimulate endocytic internalization of
ubiquitinated cargo into MVBs (Lucero et al., 2000; Kim et al., 2007).
However, the existence of K63-linked chains also proved to be
particularly important during the MVB sorting of CPS (Lucero et al.,
2000; Erpapazoglou et al., 2008; Kim et al., 2009; Lauwers et al.,
2010). Similarly, this phenomenon has been reported inArabidopsis where K63 is the second most abundant polyubiquitin
chain after K48 (Kasai et al., 2011; Lu et al., 2011; Leitner et al.,
2012; Martins et al., 2015; Dubeaux et al., 2018). Furthermore, defects
in the polyubiquitin K63 link chains from M. oryzae are alluded
to cause substantial alteration on fungal growth and development as well
as morphological changes (Oh et al., 2012).