Differences in N-linked glycosylation between human surfactant protein-B variants of the C or T allele at the single-nucleotide polymorphism at position 1580: Implications for disease

Human surfactant protein-B (SP-B), a hydrophobic protein, is essential for normal lung function. SP-B is expressed and secreted by specific lung cell types, i.e. alveolar type II and Clara cells, of the respiratory epithelium. The SP-B precursor (42 kDa) undergoes post-translational processing to generate an 8 kDa mature SP-B. A single-nucleotide polymorphism (SNP) at nucleotide 1580 (C/T) in exon 4 of SP-B that changes amino acid 131 from threonine to isoleucine (Thr¹³¹ → Ile) is associated with several pulmonary diseases. The Thr¹³¹ → Ile substitution can eliminate a potential N-linked glycosylation site, Asn¹²⁹-Gln-Thr¹³¹, which is present in the SP-B variant of the C allele (ACT/Thr) but not in that of the T allele (ATT/Ile). To determine whether the C allele SP-B variant is indeed glycosylated at Asn¹²⁹-Gln-Thr¹³¹, we first generated stably transfected Chinese hamster ovary cell lines that expressed each version of SP-B, and developed specific SP-B polyclonal anti-peptide antibodies. Using both the stably transfected cell lines and fetal lung explants, we observed that the C allele variant is indeed glycosylated at the Asn¹²⁹-Gln-Thr¹³¹ site, whereas the T allele variant, which served as a control, is not. In addition, we also confirmed that both SP-B variants contain another N-linked glycosylation site, Asn³¹¹-Ser-Ser³¹³. Given its association with several pulmonary diseases, this finding provides useful information for future studies in disease systems associated with this SNP. Further, we speculate that, given the fact that this SNP is found frequently in the general population, N-linked glycosylation at residue Asn¹²⁹ interferes with SP-B processing, secretion and folding under certain disease conditions.