The ubiquitin fold modifier 1 (Ufm1) is the most recently discovered ubiquitin-like modifier whose conjugation (ufmylation) system is conserved in multicellular organisms. Ufm1 is known to covalently attach with cellular protein(s) via a specific E1-activating enzyme (Uba5) and an E2-conjugating enzyme (Ufc1), but its E3-ligating enzyme(s) as well as the target protein(s) remain unknown. Herein, we report both a novel E3 ligase for Ufm1, designated Ufl1, and an Ufm1-specific substrate ligated by Ufl1, C20orf116. Ufm1 was covalently conjugated with C20orf116. Although Ufl1 has no obvious sequence homology to any other known E3s for ubiquitin and ubiquitin-like modifiers, the C20orf116⅐Ufm1 formation was greatly accelerated by Ufl1. The C20orf116⅐Ufm1 conjugate was cleaved by Ufm1-specific proteases, implying the reversibility of ufmylation. The conjugation was abundant in the liver and lungs of Ufm1-transgenic mice, fractionated into membrane fraction, and impaired in Uba5 knock-out cells. Intriguingly, immunological analysis revealed localizations of Ufl1 and C20orf116 mainly to the endoplasmic reticulum. Our results provide novel insights into the Ufm1 system involved in cellular regulation of multicellular organisms.

Biological roles for UFM1, a ubiquitin-like protein, are largely unknown, and therefore we screened for targets of ufmylation. Here we show that ufmylation of the nuclear receptor coactivator ASC1 is a key step for ERα transactivation in response to 17β-estradiol (E2). In the absence of E2, the UFM1-specific protease UfSP2 was bound to ASC1, which maintains ASC1 in a nonufmylated state. In the presence of E2, ERα bound ASC1 and displaced UfSP2, leading to ASC1 ufmylation. Polyufmylation of ASC1 enhanced association of p300, SRC1, and ASC1 at promoters of ERα target genes. ASC1 overexpression or UfSP2 knockdown promoted ERα-mediated tumor formation in vivo, which could be abrogated by treatment with the anti-breast cancer drug tamoxifen. In contrast, expression of ufmylation-deficient ASC1 mutant or knockdown of the UFM1-activating E1 enzyme UBA5 prevented tumor growth. These findings establish a role for ASC1 ufmylation in breast cancer development by promoting ERα transactivation.

Defining the molecular strategies that integrate diverse signalling pathways in the expression of specific gene programmes that are critical in homeostasis and disease remains a central issue in biology. This is particularly pertinent in cancer biology because downregulation of tumour metastasis suppressor genes is a common occurrence, and the underlying molecular mechanisms are not well established. Here we report that the downregulation of a metastasis suppressor gene, KAI1, in prostate cancer cells involves the inhibitory actions of beta-catenin, along with a reptin chromatin remodelling complex. This inhibitory function of beta-catenin-reptin requires both increased beta-catenin expression and recruitment of histone deacetylase activity. The coordinated actions of beta-catenin-reptin components that mediate the repressive state serve to antagonize a Tip60 coactivator complex that is required for activation; the balance of these opposing complexes controls the expression of KAI1 and metastatic potential. The molecular mechanisms underlying the antagonistic regulation of beta-catenin-reptin and the Tip60 coactivator complexes for the metastasis suppressor gene, KAI1, are likely to be prototypic of a selective downregulation strategy for many genes, including a subset of NF-kappaB target genes.

Analysis of nucleotide binding induced conformational changes in the current and previous HslU structures suggests a protein unfolding-coupled translocation mechanism. In this mechanism, unfolded polypeptides are threaded through the aligned pores of the ATPase and peptidase and translocated into the peptidase central chamber.

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