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J. Biol. Chem., Vol. 276, Issue 17, 13949-13956, April 27, 2001

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Unique Biochemical Nature of Carp Retinol-binding Protein
N-LINKED GLYCOSYLATION AND UNCLEAVABLE NH₂-TERMINAL SIGNAL PEPTIDE^*

Diana Bellovino, Takashi Morimoto^§, Elena Mengheri, Giuditta Perozzi, Ivana Garaguso, Fabio Nobili, and Sancia Gaetani^¶

From the  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

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 GenBank^TM/EMBL Data Bank with accession number(s) AJ277123.

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