Conserved cassette structure of vertebrate Mr 300 kDa mannose 6-phosphate receptors: partial cDNA sequence of fish...

The existence of two homologous mannose 6-phosphate receptors (MPRs) with overlapping, but distinct functions has raised the question of at what stage in the phylogenetic tree the two receptors have occurred for the first time. In this paper, we present a partial cDNA sequence of Mr 300 kDa MPR (MPR 300) from poeciliid fish (Xiphophorus). It contains a 5'-untranslated region followed by the initiator ATG, and an open reading frame that corresponds to cassettes 1-5 and part of cassette 6 of mammalian MPR 300. The size of the mRNA transcript for fish MPR 300 was comparable with that of other vertebrates. The amino acid sequence of fish MPR 300 displays 48-52% similarity with mammalian and chicken MPR 300. In particular, all the cysteine residues involved in disulfide bonding and an arginine residue, which is considered to be part of the mannose 6-phosphate binding site in cassette 3 of mammalian MPR 300, are conserved. Sequence similarities were significantly higher within cassette 3 and within cassette 5, to which a ligand-binding function has not yet been ascribed. Sequence similarities of the internal cassettes of MPR 300 are discussed with regard to the multifunctional nature of MPR 300.