Fucosyltransferases 12, 13: Protein O-Fucosyltransferases 1 and 2 (POFUT1, POFUT2)
Protein O-fucosyltransferases (POFUTs) are responsible for the addition of an α-linked l-fucose to the hydroxyl group of a serine or threonine residue within a particular consensus sequence in an epidermal growth factor (EGF)-like repeat (added by POFUT1) or thrombospondin type 1 repeat (TSR, added by POFUT2). Each enzyme is specific for the particular motif: POFUT1 does not modify TSRs and POFUT2 does not modify EGF repeats (Luo et al. 2006b). EGF repeats and TSRs are both small protein motifs containing six cysteines forming three disulfide bonds, although TSRs are slightly larger than EGF repeats (∼60 vs. ∼40 amino acids) and the disulfide bonding patterns are different (Adams and Tucker 2000; Campbell and Bork 1993). They are present in hundreds of cell-surface and secreted proteins, frequently in multiple tandem repeats. The presence of putative consensus sequences for O-fucosylation (C2-X-X-X-X-(S/T)-C3 for EGF repeats and C2-X-X-(S/T)-C3-X-X-G for TSRs, where Cx refers to the position of one of the conserved cysteines within the domain (Gonzalez de Peredo et al. 2002; Rana and Haltiwanger 2011; Shao et al. 2003; Wang et al. 2007)), allows prediction of which motifs will be modified. Database searches with these sequences reveal over 100 proteins with EGF repeats predicted to be modified by POFUT1 (Rampal et al. 2007) and over 50 proteins with TSRs predicted to be modified by POFUT2 (Du et al. 2010). POFUT1 and POFUT2 are both soluble enzymes localized in the endoplasmic reticulum (ER) (Luo and Haltiwanger 2005; Luo et al. 2006a; Okajima et al. 2005), and they both require a properly folded acceptor substrate (EGF repeat or TSR, respectively) for activity (Luo et al. 2006b; Wang and Spellman 1998). This ability to distinguish between folded and unfolded structures coupled with their ER localization has led to the proposal that Pofut1 and Pofut2 function as chaperones or in some aspect of quality control in the folding of their target proteins (Luo et al. 2006b; Luther and Haltiwanger 2009).
KeywordsConsensus Sequence Notch Signaling Chinese Hamster Ovary Cell Threonine Residue Notch Activity
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