MG-160, a membrane sialoglycoprotein of the medial cisternae of the rat Golgi apparatus, binds basic fibroblast growth factor and exhibits a high level of sequence identity to a chicken fibroblast growth factor receptor

Abstract

We report the primary structure of MG-160, a 160 kDa membrane sialoglycoprotein residing in the medial cisternae of the Golgi apparatus of rat neurons, pheochromocytoma (PC-12), and several other cells. The cDNA encodes a polypeptide of 1,171 amino acids with an M(r) of 133,403. An intralumenal cleavable signal peptide is followed by a Pro-Gln-rich segment and 16 contiguous, approx. 60-residue-long, regularly spaced cysteine-rich segments showing sequence identities ranging from 15 to 35%. The lumenal domain is followed by a single membrane spanning domain and a short carboxy-terminal cytoplasmic tail. The protein contains 5 potential NXT glycosylation sites. The sequence of MG-160 shows no homologies with enzymes and other membrane proteins of the Golgi apparatus. MG-160 displays a so far unique feature for a membrane protein of the Golgi apparatus: namely, an upstream, open reading frame (uORF), encoding 58 amino acids, located in front of the major open reading frame (ORF). Most vertebrate mRNAs containing uORF or AUG codons in front of the major ORF encode growth factors and cell surface receptors (Geballe and Morris 1994). In that regard a 90% identity between the primary structure of MG-160 and a receptor for acidic and basic fibroblast growth factors (CFR), isolated from chicken embryos (Burrus et. al., 1992), may be relevant. Immunoreactivity for MG-160 has been detected in the Golgi apparatus of neural and other cells of 2-day-old chicken embryos and adult chicken; furthermore, recombinant human basic fibroblast growth factor (bFGF) binds MG-160 purified from rat brain. MG-160 shows no sequence similarity with members of the family of fibroblast growth factor receptors (FGFR) involved in signal transduction. These findings are consistent with the hypothesis that MG-160 is involved in the traffic and processing of endogenous or autocrine FGFs. This is the first example of an intrinsic membrane protein of the Golgi apparatus which binds a growth factor and may be involved in its regulation.