Protein O-glucosylation: another essential role of glucose in biology

doi:10.1016/j.sbi.2018.12.001

Current Opinion in Structural Biology

Volume 56, June 2019, Pages 64-71

https://doi.org/10.1016/j.sbi.2018.12.001 Get rights and content

Highlights

•
Protein O-glucosylation on EGF repeats is essential for Notch signaling.
•
POGLUT1 recognizes the properly folded EGF repeats and O-gluco sylates the serine within the consensus sequence C¹-X-S-X-(P/A)-C².
•
XXYLT1 adds a terminal α3-linked Xyl to Xyl-Glc disaccharides on EGF repeats by an S_Ni-like retaining mechanism.
•
O-Fuc and O-Glc glycans stabilize EGF repeats, thereby regulating Notch trafficking.

Protein O-glucosylation is an unusual, linear trisaccharide form of O-glycosylation, xyloseα1-3xyloseα1-3glucose1β-O-serine, that is attached to epidermal growth factor-like (EGF) repeats found on numerous proteins including Notch. Genetic and biochemical studies have shown that protein O-glucosylation is essential for full Notch activity in Drosophila and mice. Aberrant protein O-glucosylation has been linked to human diseases. Structural studies of the glycosyltransferases, POGLUT1 and XXYLT1, in complex with substrates revealed the biosynthetic mechanisms of protein O-glucosylation. Very recently, two novel protein O-glucosyltransferases that modify sites distinct from POGLUT1 were identified. Furthermore, protein O-glucosylation turned out to modulate the stability of EGF repeats and thereby regulate Notch trafficking.

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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View full text

Protein O-glucosylation: another essential role of glucose in biology

Highlights

Graphical abstract

Introduction

Section snippets

Structures of O-glucosylation enzymes and products

Effects of O-Glc glycans on EGF stability and Notch signaling

Conclusions

Conflict of interest statement

References and recommended reading

Acknowledgements

Biochem Biophys Res Commun

Cell

Br J Dermatol

J Biol Chem

PLoS Genet

Nat Commun

Science

Annu Rev Biochem

Curr Biol

Nat Rev Mol Cell Biol

Seikagaku

Regulation of mammalian Notch signaling and embryonic development by the protein O-glucosyltransferase Rumi

Development

Protein O-glucosyltransferase 1 (POGLUT1) promotes mouse gastrulation through modification of the apical polarity protein CRUMBS2

PLoS Genet

Mutations in POGLUT1, encoding protein O-glucosyltransferase 1, cause autosomal-dominant Dowling-Degos disease

Am J Hum Genet

Protein O-fucosyltransferase 1 is an essential component of Notch signaling pathways

Proc Natl Acad Sci U S A

A POGLUT1 mutation causes a muscular dystrophy with reduced Notch signaling and satellite cell loss

EMBO Mol Med

Overexpression of human CAP10-like protein 46 KD in T-acute lymphoblastic leukemia and acute myelogenous leukemia

Genet Test Mol Biomark

Human CAP10-like protein 46 kDa gene promotes malignancy in colorectal cancer

OMICS

RUMI is a novel negative prognostic marker and therapeutic target in non-small-cell lung cancer

J Cell Physiol

Identification of glycosyltransferase 8 family members as xylosyltransferases acting on O-glucosylated notch epidermal growth factor repeats

J Biol Chem