Deglycosylating enzymes acting on N- glycans in fungi: Insights from a genome survey

NGLY1: A fascinating, multifunctional molecule

NGLY1, a cytoplasmic de-N-glycosylating enzyme is well conserved among eukaryotes. This enzyme has attracted considerable attention after mutations on the NGLY1 gene were found to cause a rare genetic disorder called NGLY1 deficiency. Recent explosive progress in NGLY1 research has revealed multi-functional aspects of this protein.

Fungal Glycosidases in Sporothrix Species and Candida albicans

Glycoside hydrolases (GHs) are enzymes that participate in many biological processes of fungi and other organisms by hydrolyzing glycosidic linkages in glycosides. They play fundamental roles in the degradation of carbohydrates and the assembly of glycoproteins and are important subjects of studies in molecular biology and biochemistry. Based on amino acid sequence similarities and 3-dimensional structures in the carbohydrate-active enzyme (CAZy), they have been classified in 171 families. Members of some of these families also exhibit the activity of trans-glycosydase or glycosyl transferase (GT), i.e., they create a new glycosidic bond in a substrate instead of breaking it. Fungal glycosidases are important for virulence by aiding tissue adhesion and colonization, nutrition, immune evasion, biofilm formation, toxin release, and antibiotic resistance. Here, we review fungal glycosidases with a particular emphasis on Sporothrix species and C. albicans, two well-recognized human pathogens. Covered issues include a brief account of Sporothrix, sporotrichosis, the different types of glycosidases, their substrates, and mechanism of action, recent advances in their identification and characterization, their potential biotechnological applications, and the limitations and challenges of their study given the rather poor available information.

Assay for the peptide:N-glycanase/NGLY1 and disease-specific biomarkers for diagnosing NGLY1 deficiency

Cytosolic peptide:N-glycanase (NGLY1 in mammals), a highly conserved enzyme in eukaryotes, catalyzes the deglycosylation of N-glycans that are attached to glycopeptide/glycoproteins. In 2012, an autosomal recessive disorder related to the NGLY1 gene, which was referred to as NGLY1 deficiency, was reported. Since then, more than 100 patients have been identified. Patients with this disease exhibit various symptoms, including various motor deficits and other neurological problems. Effective therapeutic treatments for this disease, however, have not been established. Most recently, it was demonstrated that the intracerebroventricular administration of an adeno-associated virus 9 vector expressing human NGLY1 during the weaning period allowed some motor functions to be recovered in Ngly1-/- rats. This observation led us to hypothesize that a therapeutic intervention for improving these motor deficits or other neurological symptoms found in the patients might be possible. To achieve this, it is critical to establish robust and facile methods for assaying NGLY1 activity in biological samples, for the early diagnosis and evaluation of the therapeutic efficacy for the treatment of NGLY1 deficiency. In this mini-review, we summarize progress made in the development of various assay methods for NGLY1 activity, as well as a recent progress in the identification of NGLY1 deficiency-specific biomarkers.

Folding and Quality Control of Glycoproteins

Folding of proteins is essential so that they can exert their functions. For proteins that transit the secretory pathway, folding occurs in the endoplasmic reticulum (ER) and various chaperone systems assist in acquiring their correct folding/subunit formation. N-glycosylation is one of the most conserved posttranslational modification for proteins, and in eukaryotes it occurs in the ER. Consequently, eukaryotic cells have developed various systems that utilize N-glycans to dictate and assist protein folding, or if they consistently fail to fold properly, to destroy proteins for quality control and the maintenance of homeostasis of proteins in the ER.

Mechanisms of productive folding and endoplasmic reticulum-associated degradation of glycoproteins and non-glycoproteins

BACKGROUND

The quality of proteins destined for the secretory pathway is ensured by two distinct mechanisms in the endoplasmic reticulum (ER): productive folding of newly synthesized proteins, which is assisted by ER-localized molecular chaperones and in most cases also by disulfide bond formation and transfer of an oligosaccharide unit; and ER-associated degradation (ERAD), in which proteins unfolded or misfolded in the ER are recognized and processed for delivery to the ER membrane complex, retrotranslocated through the complex with simultaneous ubiquitination, extracted by AAA-ATPase to the cytosol, and finally degraded by the proteasome.

SCOPE OF REVIEW

We describe the mechanisms of productive folding and ERAD, with particular attention to glycoproteins versus non-glycoproteins, and to yeast versus mammalian systems.

MAJOR CONCLUSION

Molecular mechanisms of the productive folding of glycoproteins and non-glycoproteins mediated by molecular chaperones and protein disulfide isomerases are well conserved from yeast to mammals. Additionally, mammals have gained an oligosaccharide structure-dependent folding cycle for glycoproteins. The molecular mechanisms of ERAD are also well conserved from yeast to mammals, but redundant expression of yeast orthologues in mammals has been encountered, particularly for components involved in recognition and processing of glycoproteins and components of the ER membrane complex involved in retrotranslocation and simultaneous ubiquitination of glycoproteins and non-glycoproteins. This may reflect an evolutionary consequence of increasing quantity or quality needs toward mammals.

GENERAL SIGNIFICANCE

The introduction of innovative genome editing technology into analysis of the mechanisms of mammalian ERAD, as exemplified here, will provide new insights into the pathogenesis of various diseases.

Signaling through plant lectins: modulation of plant immunity and beyond

Lectins constitute an abundant group of proteins that are present throughout the plant kingdom. Only recently, genome-wide screenings have unraveled the multitude of different lectin sequences within one plant species. It appears that plants employ a plurality of lectins, though relatively few lectins have already been studied and functionally characterized. Therefore, it is very likely that the full potential of lectin genes in plants is underrated. This review summarizes the knowledge of plasma membrane-bound lectins in different biological processes (such as recognition of pathogen-derived molecules and symbiosis) and illustrates the significance of soluble intracellular lectins and how they can contribute to plant signaling. Altogether, the family of plant lectins is highly complex with an enormous diversity in biochemical properties and activities.