Planarian Cytochrome b561: Conservation of a Six Transmembrane Structure and Localization along the Central and Peripheral Nervous System.

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Cytochrome b561 is a major transmembrane protein of catecholamine and neuropeptide secretory vesicles in the central and peripheral nervous systems of higher animals. We succeeded in cloning a full-length cDNA encoding planarian cytochrome b561. The deduced amino acid sequence shows a very similar six transmembrane topology to those of cytochromes b561 of higher vertebrates and contains both putative ascorbate- and monodehydro ascorbate-binding sites. Among the six totally-conserved His residues of cytochrome b561 in higher vertebrates, one is substituted with an Asn residue, indicating that His 88 and His 161 of bovine cytochrome b561 play roles as heme b ligands at the extravesicular side. Northern- and Western-blot analyses confirmed the expression of the mRNA and protein with the expected sizes in planarians. The distributions of the mRNA and apoprotein were analyzed by in situ hybridization and immunohistochemical staining, respectively, showing two morphologically distinct structures, a pair of ventral nerve cords and the cephalic ganglion cluster in the head region. The present results suggest that the usage of ascorbate to supply electron equivalents to neuroendocrine-specific copper-containing monooxygenases is likely to have originated in organisms with a very simple nervous system.
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