Abstract
The majority of E- and P-selectin ligands in leukocytes and myelocytic or monocytic leukemia cells are carried by transmembrane glycoproteins having a tandem repeat mucin-like domain through which O-linked carbohydrate ligands are carried. However, determination of structure and adhesive function of carbohydrates in glycoproteins is extremely difficult because of the extensive structural heterogeneity and the scarcity of material for functional analysis. We have overcome this difficulty through use of poly-LacNAc gangliosides isolated from a large quantity of ( approximately 1.2 L packed) HL60 cells [Stroud, M. R., Handa, K., Salyan, M. E. K., Ito, K., Levery, S. B., Hakomori, S., Reinhold, B. B., & Reinhold, V. N. (1996) Biochemistry 35, 758-769, 770-778]. We identified two major types of poly-LacNAc gangliosides without the sialosyl-Lex epitope as being capable of binding to E-selectin: (i) those having a single alpha1-->3 fucosylation at internal GlcNAcs but not at the penultimate GlcNAc and (ii) those having double alpha1-->3 fucosylation at internal GlcNAcs, excluding the penultimate GlcNAc. Gangliosides from group i above did not show any adhesion under static conditions, but showed strong adhesion under dynamic flow conditions. Gangliosides from group ii above showed adhesion under both static and dynamic conditions, as did sialosyl-Lex (SLex)-containing structures in previous studies. However, SLex-containing poly-LacNAc gangliosides are virtually absent or present in only trace quantities in leukocytes and HL60 cells. Poly-LacNAc gangliosides from groups i and ii above, lacking SLex structure, are the major membrane components of leukocytes and HL60 cells. These carbohydrates, bound to lipid or to protein, may therefore be the physiological epitope for E-selectin-dependent binding of these cells, particularly under dynamic flow conditions.
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