O -glycosylation of Notch epidermal growth factor-like (EGF) repeats has long been studied in the context of protein folding, secretion from cells, and protein function. Originally, the protein O -glucosyltransferase, POGLUT1, was thought to be the only enzyme adding an O -glucose modification to EGFs, specifically to a serine between cysteine one and two of a six cysteine containing EGF repeat. The POGLUT1 O -glucose can be elongated on Notch EGFs with xyloses by GXYLT1/2 and XXYLT1, forming a trisaccharide. Mouse knockouts of Poglut1 are embryonic lethal with Notch1 -related phenotypes. Recently, protein O -glucosyltransferases POGLUT2 and POGLUT3 were shown to add an O -glucose modification to a serine located between cysteine three and four, distinct from the POGLUT1 modification. This modification was first discovered on EGF11 of NOTCH1, then subsequently mapped on extracellular matrix proteins fibrillin-1 (FBN1), fibrillin-2 (FBN2), and latent transforming growth factor beta-binding protein 1 (LTBP1). Poglut2/3 double knockout mice exhibit neonatal lethality like Fbn1 or Ltbp1 knockouts. In addition, Poglut2/3 double knockout mice display syndactyly, similar to Fbn2 knockouts. These studies also showed decreased secretion of FBN1 and 2 from fibroblasts and incorporation into the extracellular matrix. Site mapping established a putative POGLUT2/3 consensus sequence: C 3 -x-N-T-x-G- S -F/Y-x-C 4 . Alanine variants at conserved residues redefined consensus to: C 3 -x-x-x-x-x- S -x-x-C 4 . Marfan syndrome (MFS) is caused by FBN1 variants, and MFS variants in the POGLUT2/3 consensus display aberrant O -glucosylation. With the neonatal lethality of mouse Poglut2/3 double knockouts and the changes in O -glucosylation caused by MFS variants, further examination of the effects of O -glucose on FBN1 function are needed. • Epidermal Growth Factor-like Repeats can be modified at 2 distinct sites with an O -linked glucose • Protein O -glucosyltransferase 1 (POGLUT1) adds an O -glucose to one site. • POGLUT1 is essential for Notch function. • POGLUT2 and 3 add an O -glucose to the other site. • Poglut2/3 double knockout mice are neonatal lethal and likely impair fibrillin function.
Kegley et al. (Wed,) studied this question.