Role of glycosylation of Notch in development

Abstract

The Notch pathway is one of the major signaling pathways required for proper development in metazoans. Notch activity is regulated at numerous levels, and increasing evidence reveals the importance of "protein glycosylation" (modification of Notch receptors with sugars) for its regulation. In this review we summarize the significance of the Notch pathway in development and the players responsible for its glycosylation, and then discuss the molecular mechanisms by which protein glycosylation may regulate Notch function.

Figure 1. O-Glycosylation of EGF repeats

Upper panel shows a single EGF repeat with the sites for addition of O-fucose, O-glucose, and O-GlcNAc. O-Fucose is attached to Ser/Thr in C²XXXX(S/T)C³ (red). O-Glucose is attached to Ser in C¹XSXPC² (blue). O-GlcNAc is attached to Ser/Thr between the fifth and sixth cysteines (green). Note that the consensus sequence of O-GlcNAc modification has not yet been proposed. Conserved cysteines are shown in light green. Disulfide bonds are shown by pink bars. Lower panel shows fully extended structures of O-fucose, O-glucose, and O-GlcNAc glycans and the glycosyltransferases responsible for their syntheses. Fucose (red triangle), GlcNAc (blue square), Galactose (yellow circle), Sialic acid (purple diamond), Glucose (blue circle), and Xylose (orange star). O-Fucose on Drosophila Notch has only been found as a disaccharide to date [35]. Xylosyltransferase(s) which adds a terminal xylose on O-glucose has not been cloned yet. GlcNAc-transferase(s) responsible for O-GlcNAc modfification of EGF repeats has not been cloned yet [32].

Figure 2. Potential O-fucose and O-glucose modification sites in the ECDs of Notch receptors

This is an updated version of the previously reported figure in [30] with current consensus sequences for O-fucose and O-glucose. Drosophila Notch (Swiss Prot #P07207), mouse Notch1 (Q01705), mouse Notch2 (O35516), mouse Notch3 (Q61982), and mouse Notch4 (P31695) are aligned based on homology between EGF repeats. Red ovals show EGF repeats with the O-fucose consensus sequence. Blue ovals show EGF repeats with the O-glucose consensus sequence. Blue and red shaded ovals show EGF repeats with both O-fucose and O-glucose consensus sequences. Open rectangles show Lin-12/Notch repeats. A black bar shows an essential domain for ligand binding.

Figure 3. S2 cleavage defect in Drosophila rumi mutants

rumi shows a temperature-sensitive defect in Notch signaling. Preliminary data suggests that O-glucose does not affect cell-surface presentation of Notch or ligand binding, but does affect the S2 cleavage of Notch at high temperatures [98]. Thus, O-glucose may function to hold the Notch ECD in a conformation required to link ligand binding to the conformational changes necessary for S2 cleavage.