Expression, crystallization, and preliminary X-ray analysis of a sialic acid-binding fragment of sialoadhesin in the presence and absence of ligand

Sialoadhesin is a macrophage-restricted cell surface receptor, consisting of 17 immunoglobulin domains, which mediates cell adhesion via the recognition of specific sialylated glycoconjugates. A functional fragment of sialoadhesin, comprising the N-terminal immunoglobulin domain, has been expressed in Chinese hamster ovary cells as both native (SnD1) and selenomethionyl (Se-SnD1) stop protein. The successful production of 86% selenomethionine-incorporated protein represents a rare example of production of selenium-labeled protein in mammalian cells. SnD1 and Se-SnD1 have been crystallized in the absence of ligand, and SnD1 has also been crystallized in the presence of its ligand 2,3 sialyllactose. The ligand-free crystals of SnD1 and Se-SnD1 were isomorphous, of space group P3₁21 or P3₂21, with unit cell dimensions a = b = 38.9 Å, c = 152.6 Å, α = β = 90°, γ 120°, and diffracted to a maximum resolution of 2.6 Å. Cocrystals containing 2,3 sialyllactose diffracted to 1.85 Å at a synchrotron source and belong to space group P2₁2₁2₁, with unit cell dimensions α = 40.9 Å, b = 97.6 Å, c = 101.6 Å, α = β = υ = 90°.