The disulfide catalyst QSOX1 maintains the colon mucosal barrier by regulating Golgi glycosyltransferases.

Mucus is made of enormous mucin glycoproteins that polymerize by disulfide crosslinking in the Golgi apparatus. QSOX1 is a catalyst of disulfide bond formation localized to the Golgi. Both QSOX1 and mucins are highly expressed in goblet cells of mucosal tissues, leading to the hypothesis that QSOX1 catalyzes disulfide-mediated mucin polymerization.

Disulfide bond formation and redox regulation in the Golgi apparatus.

Formation of disulfide bonds in secreted and cell-surface proteins involves numerous enzymes and chaperones abundant in the endoplasmic reticulum (ER), the first and main site for disulfide bonding in the secretory pathway. Although the Golgi apparatus is the major station after the ER, little is known about thiol-based redox activity in this compartment. QSOX1 and its paralog QSOX2 are the only known Golgi-resident enzymes catalyzing disulfide bonding.

Intestinal mucin is a chaperone of multivalent copper.

Mucus protects the epithelial cells of the digestive and respiratory tracts from pathogens and other hazards. Progress in determining the molecular mechanisms of mucus barrier function has been limited by the lack of high-resolution structural information on mucins, the giant, secreted, gel-forming glycoproteins that are the major constituents of mucus. Here, we report how mucin structures we determined enabled the discovery of an unanticipated protective role of mucus: managing the toxic transition metal copper.

Conformational switches and redox properties of the colon cancer-associated human lectin ZG16.

Zymogen granule membrane protein 16 (ZG16) is produced in organs that secrete large quantities of enzymes and other proteins into the digestive tract. ZG16 binds microbial pathogens, and lower ZG16 expression levels correlate with colorectal cancer, but the physiological function of the protein is poorly understood. One prominent attribute of ZG16 is its ability to bind glycans, but other aspects of the protein may also contribute to activity.

Phenotypic Screen Identifies JAK2 as a Major Regulator of FAT10 Expression.

FAT10 is a ubiquitin-like protein suggested to target proteins for proteasomal degradation. It is highly upregulated upon pro-inflammatory cytokines, namely, TNFα, IFNγ, and IL6, and was found to be highly expressed in various epithelial cancers. Evidence suggests that FAT10 is involved in cancer development and may have a pro-tumorigenic role.

Intestinal Gel-Forming Mucins Polymerize by Disulfide-Mediated Dimerization of D3 Domains.

The mucin 2 glycoprotein assembles into a complex hydrogel that protects intestinal epithelia and houses the gut microbiome. A major step in mucin 2 assembly is further multimerization of preformed mucin dimers, thought to produce a honeycomb-like arrangement upon hydrogel expansion. Important open questions are how multiple mucin 2 dimers become covalently linked to one another and how mucin 2 multimerization compares with analogous processes in related polymers such as respiratory tract mucins and the hemostasis protein von Willebrand factor.

Rapid Covalent-Probe Discovery by Electrophile-Fragment Screening.

Covalent probes can display unmatched potency, selectivity, and duration of action; however, their discovery is challenging. In principle, fragments that can irreversibly bind their target can overcome the low affinity that limits reversible fragment screening, but such electrophilic fragments were considered nonselective and were rarely screened. We hypothesized that mild electrophiles might overcome the selectivity challenge and constructed a library of 993 mildly electrophilic fragments.