Characterization of glycosaminoglycan modifications and a related galactosyltransferase B4GALT7 in Bombyx mori.

Glycosaminoglycan (GAG) modifications mediate multiple biological processes, yet the GAG modification landscape remains poorly characterized, especially in invertebrates. The silkworm Bombyx mori is an important economic insect, a lepidopteran model species, and a promising bioreactor for recombinant protein production. Herein, we profiled the GAG modifications in Bombyx mori and identified 21 chondroitin sulfate (CS)-modified proteoglycans. Bioinformatic analyses indicate that several of these proteoglycans participate in cytoskeletal organization, cell adhesion, and immune response, whereas the majority still lack sufficient functional annotation or reference information from homologs. We further characterized silkworm B4GALT7, a core enzyme participating in the biosynthesis of the tetrasaccharide linkage of GAG, and demonstrated that B4GALT7 could positively regulate global GAG modification levels in silkworm cells. Silkworm B4GALT7 localizes to the Golgi apparatus and displays broad substrate tolerance toward several sugar donors and diverse acceptors including xylose, pNP-sugars and phenolic compounds, exhibiting enzymatic features distinct from its human and Drosophila homologs. Its optimal catalytic conditions were defined as 25 °C, pH 7.0, and 1 mM Ca2+. This study establishes the first CS-proteoglycan profiles in B. mori, and characterized a GAG biosynthetic enzyme, B4GALT7, providing a fundamental basis for further exploration of the structures and functions of GAG modification in this model insect.