Heterogeneous xylose-rich glycans are associated with extracellular glycoproteins from the biofouling diatom Craspedostauros australis (Bacillariophyceae)

Abstract

Extracellular, high-molecular-weight glycoconjugates were extracted from cleaned frustules of Craspedostauros australis with 8 M urea in 50% formic acid (UFE) or with 2 M guanidine-HCl (GU). The weight ratio of protein : carbohydrate : sulphate : phosphate was 42 : 8 : 3 : 1 in the GU extract. The three dominant molecules in the extracts (one of 112 kDa and two > 220 kDa) showed slightly increased mobility by SDS-polyacrylamide gel electrophoresis following deglycosylation with anhydrous HF, demonstrating that they were glycoproteins. The carbohydrates were studied by a combination of carboxyl-reduction of uronic acids, N-acetylation of aminosugars, and characterization of the constituent sugars as alditol acetates or as trimethyl silyl derivatives of their corresponding methyl glycosides. Sixteen sugars were identified in the extracts, including two pentoses, four hexoses, two 6-deoxyhexoses, five mono-O-methylated sugars, two uronic acids, and one amino sugar. The predominant sugar was xylose (37 – 41 mol%), with significant proportions of galactose (13 – 15 mol%), rhamnose (10 – 15 mol%), and mannose (10 mol%) also present. The linkage and substitution patterns of the sugars characterized by methylation analysis were complex. The predominant residue was terminal xylopyranose (16 mol%) followed by 2- and 4-linked xylopyranose (8 and 7 mol%, respectively). Terminal residues outnumbered the available branch points by ca. 4 : 1, indicating that the carbohydrates were mainly short-chain oligosaccharides. Together with previous studies of immunolocalization and analysis of physical properties, the composition of the C. australis glycoproteins suggests that they serve mucin-like functions on the cell surface but also supports the proposal that glycoproteins are involved in diatom cell adhesion.