The UFMylation System in Proteostasis and Beyond

Abstract

Post-translational modifications are at the apex of cellular communication and eventually regulate every aspect of life. The identification of new post-translational modifiers is opening alternative avenues in understanding fundamental cell biology processes and may ultimately provide novel therapeutic opportunities. The ubiquitin-fold modifier 1 (UFM1) is a post-translational modifier discovered a decade ago but its biological significance has remained mostly unknown. The field has recently witnessed an explosion of research uncovering the implications of the pathway to cellular homeostasis in living organisms. We overview recent advances in the function and regulation of the UFM1 pathway, and its implications for cell physiology and disease.

Keywords: ER stress; UBL; UFM1; UPR; proteostasis.

Figure 1.. The UFM1 conjugation pathway.

The zymogen pro-UFM1 is processed by the protease UFSP2 in the C-terminal, which exposes a glycine residue and yield the mature UFM1 group. UBA5 activates UFM1 through an adenylation reaction that consume ATP. UFC1, the E2 conjugating enzyme, retrieves UFM1 from UBA5 by forming a transthioester bond with UFM1. Next, UFL1 recruits an ufmylation substrate and the charged UFC1. This complex is anchored at the cytosolic side of the ER, UFL1 spanning the membrane and UFBP1 displaying a short transmembrane tail domain. CDK5RAP3 (CDK5R3 on the Figure) acts as a stabilizer for the complex at the ER. Both UFBP1 and CDK5RAP3 are qualified as possible adaptor proteins, allowing the ligase UFL1 to recruit a wider pool of substrates. Eventually, the substrates become mono- or poly-ufmylated and released from the complex. UFSP2 also operates as a major deufmylation enzyme of the pathway, removing UFM1 molecules from their targets.

Figure 2, Key Figure.. The interplay between the UFM1 pathway, ER stress and cell fate.

The ufmylation pathway can control different physiological processes depending on the stimuli and cellular background. A. The UFM1 pathway is canonically associated with the ER stress response (UPR, See Box 2 for details) and its expression is controlled by the major transcription factor XBP1s. The genetic deletion of the pathway engages ER-dependent apoptosis in many different background and its expression helps to cope with pharmacologically induced ER stress. B. Plasma B-Cell differentiation is activated by exposure to LPS, which induces a transcriptional increase in UFM1 pathway interactor UFBP1 through XBP1s expression. UFBP1 represses the activation of the sensors IRE1 and PERK as a feedback response and promotes ER expansion, allowing immunoglobulin production. C. In breast cancer cell lines, exposure to estradiol (E₂) promotes the interaction of the nuclear receptor ERα to ASC1 and subsequent poly-ufmylation. ASC1 ufmylation forms a scaffold to recruit p300 and SRC and activate cell proliferation. ASC1 is also a critical regulator of hematopoietic stem cells differentiation, and genetic deletion of ufmylation components is correlated with pancytopenia in a genetic mouse model. Overall, the ufmylation pathway appears to regulate cell fate either by reducing or preventing ER stress, or through transcriptional programs.

Figure 3.. Control of proteostasis by the ufmylation pathway.

RPL26 is one of the main ufmylation substrates. This ufmylation event tunes ER ribosome turnover and controls biogenesis of secreted proteins. In addition, deficiency in ufmylation feeds back to alter the expression of transcripts of extracellular matrix components. The mechanisms behind this regulation are unknown but could result in further imbalance in ER protein load because collagen proteins are the most abundant ER cargo. Another way for ufmylation to control ER stress would be through the targeting of transcription factors involved in ER proteostasis. Indeed ufmylation events at the nucleus are possible following the relocalization of UFL1. The remodeling of chromatin by ufmylation to alter transcriptional programs also appears as an exciting possibility. Histone ufmylation has been connected to the DNA damage response but it is likely that other functions are related to this type of regulation. Lastly, ufmylation components such as UFBP1 promote ER expansion and reorganization, which could be involved in the collapse and cell death observed in cells with high secretory activity and deficient for ufmylation components.

Figure I.

Text Box 1. Ubl conjugation enzymatic reaction.

Figure I.

Text Box 2. The unfolded protein response.