POGLUT2 and POGLUT3: Two essential protein O-glucosyltransferases modifying EGF repeats in extracellular matrix proteins

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: C³-x-N-T-x-G-S-F/Y-x-C⁴. Alanine variants at conserved residues redefined consensus to: C³-x-x-x-x-x-S-x-x-C⁴. 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.