Protein O-glucosylation: another essential role of glucose in biology
Graphical abstract
Introduction
O-Linked glucose (Glc) structures were originally found on epidermal growth factor-like (EGF) repeats of coagulation factors and then Notch; however, the biological significance of O-glucosylation had been unknown for two decades [1]. Rumi was identified as an essential component for Notch signaling using a genetic screen in Drosophila and shown to encode protein O-glucosyltransferase 1 (POGLUT1) that modifies Notch in 2008 [2••]. POGLUT1/Rumi turned out to be evolutionarily well-conserved. Systemic deletion of Poglut1 in mice causes an embryonic lethality not only with Notch-like phenotypes such as defects in neurogenesis, cardiovascular remodeling, and somitogenesis, but also with aberrant gastrulation [3,4].
Mutations in POGLUT1 that cause Dowling-Degos Disease (DDD) have been found [5]. DDD is an autosomal-dominant disorder of skin pigmentation. DDD patients have also been reported with mutations in POFUT1, KRT5, or PSENEN [6]. Although the precise mechanism of the pathogenesis of DDD is still unclear, aberrant Notch signaling may be involved. Heterozygous deletion of Pofut1 or Poglut1 in mice does not cause DDD-like phenotypes [3,7]. The discrepancy might be due to difference of species. Further analysis is in progress to reveal the pathogenic mechanism of DDD. Recently, a p.D233E mutation in POGLUT1 was identified that causes a new class of adult-onset limb-girdle muscular dystrophy with reduced Notch signaling in muscular stem cells, called satellite cells, and hypoglycosylation on α-dystroglycan in muscles [8]. Aberrant gene expression of POGLUT1 has been reported in different types of cancer [9, 10, 11].
O-Glc on EGF repeats added by POGLUT1 can be extended with two xylose (Xyl) residues in an α3-linkage (Figure 1a). The first and second Xyl residues are added by two glucoside xylosyltransferases (GXYLT1 and GXYLT2) and xyloside xylosyltransferase 1 (XXYLT1), respectively, in mammals [12,13]. Xylosyl-extension of O-Glc glycans negatively regulates Notch activation in Drosophila [14,15]. XXYLT1 appears to be amplified in numerous types of cancer including squamous carcinomas from lung and head and neck, in which Notch signaling has been implicated as a tumor suppressor [16••].
Here, we summarize recent advances in our understanding of protein O-glucosylation from the structural points of view.
Section snippets
Structures of O-glucosylation enzymes and products
Notch is important in multiple developmental processes and has been implicated in multiple human diseases and, therefore, the Notch pathway has been gaining increased attractions in therapeutic applications [17]. There is growing interest in the molecular mechanism of Notch modifying and regulating enzymes as the knowledge gained will greatly help to probe Notch’s role in multiple diseases [1].
Effects of O-Glc glycans on EGF stability and Notch signaling
O-Glucosylation and O-fucosylation on EGF repeats are required for full Notch activity [1]. Addition of O-Fuc is catalyzed by protein O-fucosyltransferase 1 (POFUT1) which is localized in the ER like POGLUT1 [31,32]. The crystal structures of EGF repeats 11–13 of human NOTCH1 modified with either the O-Fuc monosaccharide or the GlcNAc-Fuc disaccharide at T466 of EGF12 revealed that the O-Fuc modifications are well-ordered through the intramolecular contacts between the modifications and EGF12
Conclusions
Structural and biochemical analyses revealed the exquisite mechanistic regulation of protein O-glucosylation. POGLUT1 recognizes properly folded EGF repeats mainly through the recognition of the consensus sequence as well as the hydrophobic region formed only when the EGF repeats are folded correctly. XXYLT1 appears to utilize another feature of EGF repeats; spring-like flexibility. The resulting O-Glc trisaccharide stabilizes folded EGF repeats via intramolecular interactions with the same
Conflict of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
The original work described in this manuscript was performed in the Haltiwanger laboratory and the Li laboratory. We thank Drs. Haltiwanger and Li for critical comments on this manuscript. This work was supported by KAKENHI Start-up grant [17H06743] (to H.T.), Start-up grant from Huazhong University of Science and Technology (to H.Y.), and research grants from Takeda Science Foundation and Daiichi Sankyo Foundation of Life Science (to H.T.).
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