LIG_UFM1_UFIM_1

As with classical ubiquitination, ubiquitin-like proteins are also conjugated to their target proteins through a three-step enzymatic process. The key players of the protein ufmylation process include UFM1, UBA5, UFC1, UFL1, and UFSP1,2 (Wei,2016). UFM1 is activated by the E1 enzyme UBA5, which forms a high-energy thioester bond between its catalytic cysteine and the exposed C-terminal glycine of UFM1. The E2 ligase UFC1 then binds to UBA5 and the activated UFM1 is transferred to UFC1. Finally, the UFM1-specific ligase 1 (UFL1) acts as the E3 to recognize its substrate, transfer and ligate the UFM1 from E2 to the substrate. The reversal of ufmylation is carried out by the two UFM1-specific proteases, UFSP1 and UFSP2.
The UFM1 interaction is mediated by a short linear sequence, UFM1-interacting motif (UFIM) present in the C-terminal IDP segment of UBA5. This interaction allows UFM1 to occupy the activation site near the ATP-binding site in the adenylation domain with higher efficiency. As well as UFM1, UBA5 is also capable of binding a different family of UBLs, the autophagy-specific UBLs (LC3/GABARAP) using the UFIM peptide. The UFIM motif has some resemblance towards the canonical LIR (LC3 interacting region) motif present in proteins like selective autophagy receptors p62/SQSTM1 and Optineurin, hence the name non-canonical LIR (Rogov,2014). The UFIM motif does also interact with LC3. Though they have a similar mode of interaction, UFIM is different from the canonical LIR in the spacing between the aromatic and aliphatic residues and its unique antiparallel orientation (Habisov,2016). The dual binding modes exhibited by the LIR/UFIM peptide is so far purely based on in vitro experiments. At time of writing, in-cell experimental evidence is lacking for the interaction between UBA5 and LC3/GABARAPs and the role played by these interactions on protein ufmylation and vesicle trafficking have still to be delineated. The UFM1 core components are located at the cytosolic side of the ER membrane and play an important role in ER homeostasis. Therefore they are often associated with endoplasmic stress response associated diseases like type 2 diabetes and ischemic heart diseases (Wei,2016).