Differential splicing of the lectin domain of an O-glycosyltransferase modulates both peptide and glycopeptide preferences.

Mucin-type O-glycosylation is an essential post-translational modification required for protein secretion, extracellular matrix formation and organ growth. O-glycosylation is initiated by a large family of enzymes (GALNTs in mammals and PGANTs in Drosophila) that catalyze the addition of N-acetylgalactosamine (GalNAc) onto the hydroxyl groups of serines or threonines in protein substrates. These enzymes contain 2 functional domains; a catalytic domain and a C-terminal ricin-like lectin domain comprised of 3 potential GalNAc recognition repeats termed α, β and γ. The catalytic domain is responsible for binding donor and acceptor substrates and catalyzing transfer of GalNAc, while the lectin domain recognizes more distant extant GalNAc on previously glycosylated substrates. We previously demonstrated a novel role for the α repeat of lectin domain in influencing charged peptide preferences. Here, we further interrogate how the differentially spliced α repeat of the PGANT9A and PGANT9B O-glycosyltransferases confers distinct preferences for a variety of endogenous substrates. Through biochemical analyses and in silico modeling using preferred substrates, we find that a combination of charged residues within the α repeat and charged residues in the flexible gating loop of the catalytic domain distinctively influence the peptide substrate preferences of each splice variant. Moreover, PGANT9A and PGANT9B also display unique glycopeptide preferences. These data illustrate how changes within the non-catalytic lectin domain can alter the recognition of both peptide and glycopeptide substrates. Overall, our results elucidate a novel mechanism for modulating substrate preferences of O-glycosyltransferases via alternative splicing within specific subregions of functional domains.