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J. Biol. Chem., Vol. 276, Issue 17, 13949-13956, April 27, 2001
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From the Retinol transport and metabolism have been well
characterized in mammals; however, very little is known in fish. To
study the mechanism by which fish retinol-binding protein (RBP) is able to remain in plasma besides its small molecular size, we isolated RBP
cDNA from a carp liver cDNA library. Comparison of the deduced amino acid sequence with that of known vertebrate RBPs showed that carp
RBP has high homology to the other cloned vertebrate RBPs, but it lacks
the COOH-terminal tetrapeptide, RNL(S)L, which is most likely involved
in the interaction with transthyretin in mammalian RBPs. In addition,
the primary structure of carp RBP contains two consensus
N-linked glycosylation sites that represent a unique
feature. We have obtained experimental evidence, by in vitro and in vivo expression experiments, that both
sites are indeed glycosylated. We have also characterized the protein
as a complex type N-linked glycoprotein by lectin binding
assay, neuraminidase and endoglycosidase H and F digestion. Inhibition of glycosylation by tunicamycin treatment of transfected cells caused a
great reduction of RBP secretion. Since kidney filtration of anionic
proteins is less than half that of neutral protein of the same size,
this finding strongly suggests that the amount of carp RBP filtration
through kidney glomeruli may be reduced by a
glycosylation-dependent increase in the molecular size and negative charge of the protein. A second unique feature of carp RBP as
secretory protein is the presence of a nonconserved
NH2-terminal hydrophobic domain, which functions as an
insertion signal but is not cleaved cotranslationally and remains in
the secreted RBP.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) AJ277123.
Unique Biochemical Nature of Carp Retinol-binding
Protein
N-LINKED GLYCOSYLATION AND UNCLEAVABLE
NH2-TERMINAL SIGNAL PEPTIDE*
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¶
Istituto Nazionale di Ricerca per gli
Alimenti e la Nutrizione, Via Ardeatina 546, 00178 Rome, Italy and the
§ Department of Cell Biology, New York University, School of
Medicine, New York, New York 10016
*
This work was supported by a grant from the Italian Ministry
of Agriculture, Department of Aquaculture.The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
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