Characterization of the Subunit Structure, Degree of Glycosylation, and pH Dependence of the Serum Mannose-Binding Lectin in the Spiny Dogfish Shark (Squalus Acanthias)
Bayer, Rebecca A.
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Sharks exhibit a low incidence of cancer and infection. Exploration of the shark immune system may aid in our understanding of this phenomenon. Although sharks have only one class of antibodies as compared with the five classes of immunoglobulins in the human system, they do have a 10 fold increase of serum mannose-binding lectin (MBL) as compared with humans, rats, guinea pigs, and rabbits. Mannose-binding proteins appear to act as accessory molecules to the mammalian immune system through binding to mannosylated glycoproteins that are prevalent on the surfaces of certain bacteria, fungi, and viruses. MBL was previously detected in shark serum and found to be Ca2+ independent with a preliminary subunit molecular weight of 140 kD. This study further characterizes the shark MBL in terms of its subunit structure, degree of glycosylation, and pH dependence. MBL was isolated from serum by affinity chromatography on a mannose-Sepharose column. 50S-PAGE showed that the reduced and nonreduced molecular weights were 141 and 125 kO respectively. Thus, intrachain disulfide bonds appear to be present, but no interchain disulfide bonds link the subunits together. MBL was determined to have little to no glycosylation as treatment with Endo H resulted in no change in the subunit molecular weight. The binding of MBL to mannose-Sepharose beads was examined over a pH range of 4.0-11.0. The resulting bell-shaped curve showed maximum binding at pH 6.0-8.0 similar to the pH dependence of mannose-binding protein 2 in the human system.