To determine occupancy of N-linked glycans at each site, we employed a sequential deglycoslyation approach by using Endoglycosidase H and PNGase F in the presence of 18O-H2O after tryptic digestion of S (Wang et al., 2020; Yu et al., 2018). After LC-MS/MS analyses, the resulting data confirmed that 19 of the canonical sequons had occupancies greater than 95% (Table S5). One canonical sequence, N0149, had insufficient spectral counts for quantification by this method, but subsequent analyses described below suggested high occupancy. The two most C-terminal N-linked sites, N1173 and N1194, had reduced occupancy, 52% and 82% respectively. Reduced occupancy at these sites could reflect hindered en bloc transfer by the oligosaccharyltransferase (OST) due to primary amino acid sequences at or near the N-linked sequon. Alternatively, this could reflect these two sites being post-translationally modified after release of the protein by the ribosome by a less efficient STT3B-containing OST, either due to activity or initial folding of the polypeptide, as opposed to co-translationally modified by the STT3A-containing OST (Ruiz-Canada et al., 2009). None of the non-canonical sequons (three N-X-C sites and four N-G-L/I/V sites; Zielinska et al., 2010) showed significant occupancy (>5%), except for N0501, which showed moderate (19%) conversion to 18O-Asp that could be due to deamidation that is facilitated by glycine at the +1 position (Table S5) (Palmisano et al., 2012). Further analysis of this site (see below) by direct glycopeptide analyses allowed us to determine that N0501 undergoes deamidation but is not glycosylated. Thus, all, and only the, 22 canonical sequences for N-linked glycosylation (N-X-S/T) are utilized, with only N1173 and N1194 demonstrating occupancies below 95%.