Ubiquitin, a 76 amino acid protein (8.6 kDa), is conserved in eukaryotes but absent in bacteria and archaea. Ubiquitin conjugates to other proteins via peptide bond formation through its C-terminal glycine residue and a primary amine on the substrates, most commonly a lysine residue. The topology of the ubiquitin chains determines the fate of the substrate, for instance, Lys48-linked chains take proteins to the proteasome for degradation and Lys63-linked chains involve in various non-degradative roles (Chen and Sun, 2009, Hershko and Ciechanover, 1998, Komander and Rape, 2012, Swatek and Komander, 2016).

Ubiquitin-like proteins (UBLs) encompass a family of proteins that are structurally and evolutionary linked to ubiquitin. UBLs are involved in diverse cellular functions such as cell division, DNA repair, transcription, protein turnover, protein trafficking, autophagy, innate immunity activation, and apoptosis (Jentsch and Pyrowolakis, 2000, Kerscher et al., 2006). UBLs are generally classified into two types, type I UBLs that are covalently linked to the target molecules, and type II UBLs that contain protein domains related to ubiquitin, and these proteins are not conjugated to other proteins (Cappadocia and Lima, 2018, Chanarat and Mishra, 2018, Jentsch and Pyrowolakis, 2000). Hub1 (homologous to ubiquitin 1; UBL5/Beacon) is an unconventional member of the ubiquitin-like (UBL) family that may be considered as a type II UBLs, as its conjugation to the substrates has not been observed (Cappadocia and Lima, 2018, Lüders et al., 2003).

Hub1/UBL5, an evolutionarily conserved ubiquitin-like protein, adapts β-grasp fold similar to ubiquitin, which shares a 22% sequence identity with ubiquitin (Ramelot et al., 2003). In contrast to ubiquitin and UBLs, Hub1 lacks C-terminal diglycine residues and enzyme cascade for covalent conjugation to the protein substrates. Hub1 has conserved di-tyrosine (YY) residues just before a variable amino acid at the C-terminus which is involved in the final β-sheet of the protein (Lüders et al., 2003, McNally et al., 2003, Wilkinson et al., 2004, Yashiroda and Tanaka, 2004). Hub1 functions mainly in pre-mRNA splicing (Fig S1), the mitochondrial unfolded protein response, the Fanconi anemia (FA) DNA repair pathway, and protein degradation (Table S1) (Benedetti et al., 2006, Chanarat, 2021, Chanarat and Mishra, 2018, Chen et al., 2020, Kolathur et al., 2022, Mishra et al., 2011, Oka et al., 2015, Watanabe et al., 2019). Further, a recent Y2H interactor study suggests that Hub1 might possess broader roles besides the splicing function (Varikkapulakkal et al., 2022).