Ra. Siciliano et al., THE LEWIS-X EPITOPE IS A MAJOR NONREDUCING STRUCTURE IN THE SULFATED N-GLYCANS ATTACHED TO ASN-65 OF BOVINE PROOPIOMELANOCORTIN, Glycobiology, 3(3), 1993, pp. 225-239
The N-terminal glycopeptide of pro-opiomelanocortin (POMC), designated
as the 16K fragment, is highly conserved throughout vertebrates from
amphibians to mammals and is likely therefore to have an important fun
ctional role. In this paper, we report the first structural characteri
zation of N-glycans attached to asparagine-65 of a 16K glycopeptide. T
he 16K fragment was isolated from bovine pituitaries and the N-glycans
were analysed using fast atom bombardment mass spectrometry together
with sugar and linkage analysis. Sulphated-N-acetylgalactosamine-cappe
d antennae, typical of the pituitary glycohormones, were present in th
e major acidic components. The POMC oligosaccharides are distinct from
those of the pituitary glycohormones because the sulphate is exclusiv
ely located on the 3-arm of biantennary structures and, in addition, a
significant proportion of the molecules carry the Lewis x epitope. It
is probable that these differences reflect the absence of a tripeptid
e motif in POMC which fully conforms to the criteria previously define
d for the recognition sequence for the N-acetylgalactosamine transfera
se that is specific for the pituitary glycohormones [Smith and Baenzig
er (1992) Proc. Natl. Acad. Sci. USA, 89, 329-333]. It remains to be s
een whether the Lewis x epitope is involved in selectin-mediated event
s, but previous studies suggest that the sulphated moieties are unlike
ly to play a major role in clearance. The Lewis x epitope is also pres
ent in the neutral N-linked oligosaccharides, together with a variety
of other antennae including a rarely found fucosylated GalNAc-GlcNAc s
tructure.