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).