Our laboratory is studying the structure and function of complex carbohydrates in biological systems. This relatively new field, termed "Glycobiology", has grown rapidly with the realization that glycoconjugates play important roles in nearly all aspects of metazoan biology. The fundamental question being addressed in our laboratory is: What are the functional consequences of covalently modifying proteins with sugars, especially as related to signal transduction events in cells?
Role of O-glycosylation in the Notch signaling pathway
Recent work in our laboratory has demonstrated that signal transduction pathways, such as that controlled by the Notch receptor, can be regulated by changing the structure of the carbohydrate modifications on the receptor. Notch is a cell surface receptor that plays a key role in numerous phases of development and differentiation. Defects in Notch signaling can cause numerous developmental deformities in organisms from Drosophila to mammals, including human diseases such as T cell leukemia, a type of cerebral arteriopathy (CADASIL), Alagille syndrome, and a common form of congenital heart disease (Tetrology of Fallot). We have shown that Notch is modified with two unusual forms of O-linked glycosylation, O-fucose and O-glucose, on the epidermal growth factor-like (EGF) repeats in its extracellular domain (see Moloney et al., 2000, J. Biol. Chem. 275, 9604-9611). Most of Notch’s 36 tandem EGF repeats contain putative consensus sequences for the addition of one or the other of these sugars, and many of these sites are evolutionary conserved. Even more significantly, we have discovered a biological role for the O-fucose modifications by showing that the Fringe protein, a known modulator of Notch function, is an O-fucose specific ß1,3 N-acetylglucosaminyltransferase (see Moloney et al., 2000 Nature 406, 369-375). These results strongly suggest that Fringe mediates its affects on Notch function by altering the O-fucose structures on Notch. The modulation of Notch signaling by elongation of O-fucose provides a new paradigm for the involvement of glycosylation in signal transduction events. We have also shown that the O-glucose modifications on EGF repeats are essential for Notch function. Mutations in the enzyme responsible for addition of O-glucose to EGF repeats, Rumi, result in Notch phenotypes in flies (see Acar et al., 2008Cell 132, 247-258). We are currently examining the mechanism by which the O-fucose and O-glucose glycans affect Notch activity (see "Recent Publications" for more details).
O-Fucose modifications on Thrombospondin type 1 Repeats
O-Fucose modifications are also known to exist in the context of a different cysteine-rich motif: thrombospondin type 1 repeats (TSRs). We have recently demonstrated that the O-fucosylation of TSRs is mediated by a distinct set of enzymes than those that modify EGF repeats (see Luo et al., 2006, J. Biol. Chem. 281, 9385-9392). We have identified the enzyme responsible for addition of O-fucose to TSRs, protein O-fucosyltransferase 2 (POFUT2) (see Luo et al., 2006, J. Biol. Chem. 281, 9393-9399). Preventing addition of O-fucose to the TSRs of ADAMTS13 or ADAMTSL1 causes a reduction in their secretion, suggesting that O-fucosylation of TSRs may function in quality control or folding (see Wang et al., 2007, J. Biol. Chem. 282, 17024-17031; Ricketts et al., 2007, J. Biol. Chem. 282, 17014-17023). The fact that POFUT2 is ER-localized and only modifies properly folded TSRs adds support to this idea. We are currently examining biological functions for O-fucosylation of TSRs on other proteins.
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