Electrostatics and N‐Glycan‐mediated Membrane Tethering of SCUBE1 Is Critical for Promoting Bone Morphogenetic Protein Signaling

Abstract

SCUBE1, a secreted and membrane‐bound glycoprotein, has a modular protein structure composed of an NH2‐terminal signal peptide sequence followed by 9 epidermal growth factor (EGF)‐like repeats, a spacer region and 3 cysteine‐rich (CR) motifs with multiple potential N‐linked glycosylation sites, and one CUB domain at the COOH terminus. Soluble SCUBE1 is a biomarker of platelet activation but an active participant of thrombosis via its adhesive EGF‐like repeats, whereas its membrane form acts as a bone morphogenetic protein (BMP) co‐receptor in promoting BMP signal activity. However, the mechanism responsible for the membrane tethering and biological importance of N‐glycosylation of SCUBE1 remains largely unknown. In this study, molecular mapping analysis identified a polycationic segment (amino acids 501–550) in the spacer region required for its membrane tethering via electrostatic interactions possibly with the anionic heparan sulfate proteoglycans. Furthermore, deglycosylation by peptide‐N‐glycosidase F treatment revealed that N‐glycans within the CR motif are essential for membrane recruitment through lectin‐mediated surface retention. Injection of mRNA encoding zebrafish wild‐type but not N‐glycan–deficient scube1 restores the expression of hematopoietic and erythroid markers (scl and gata1) in scube1‐knockdown embryos. We describe novel mechanisms in targeting SCUBE1 to the plasma membrane and demonstrate that N‐glycans are required for SCUBE1 functions during primitive hematopoiesis in zebrafish.