Method for Identifying Galectin Ligands on Lymphocyte Membrane Glycoproteins.

Glycosylation is one of the most common protein posttranslational modifications. Most human lymphocyte membrane receptors are modified by diverse glycan structures, and functional studies have indicated that a family of glycan-binding proteins, galectins, can significantly modulate lymphocyte development and function by interacting with these glycans. Several galectins have a varying degree of affinity for the N-acetyllactosamine (LacNAc) disaccharide, and some critical lymphocyte receptors can be modified by glycan structures carrying this motif. However, the site-specific glycan composition on primary lymphocyte membrane receptors in healthy individuals is largely limited. The main reason for the limitation is low abundance of available material from a single donor and compositional heterogeneity in glycan structures that can potentially modify a protein. Donor-dependent variability in N-glycan structures on CD16a isolated from primary NK cells of healthy human donors was recently reported. NK cell CD16a is glycosylated at five N-glycosylation sites, and two of the five sites are modified, almost exclusively, by N-glycans with multiple LacNAc repeats which can serve as ligands for endogenous galectins. Thus, the protocol described in this section can be utilized to identify galectin ligands at specific glycosylation sites of endogenous membrane receptor from circulating primary human lymphocytes.