Xenopus cytochrome b561: molecular confirmation of a general five transmembrane structure and developmental regulation at the gastrula stage.

Cytochrome b561 transports electrons by a novel transmembrane vectoral pathway. Using the human cytochrome b561 cDNA probe, the Xenopus cytochrome b561 cDNA has been isolated and sequenced. A specific ATG was unambiguously identified because the Xenopus sequence has a stop codon 7 amino acids upstream from the initiation site ATG. The 741-bp open reading frame encodes a 247-amino-acid protein. The position of the initiating ATG site in Xenopus cytochrome b561 is consistent with the first ATG site in human and mouse proteins, and with the second of two ATG sites in the bovine protein. This result indicates that a five transmembrane domain model best represents the conformation of cytochrome b561 in membranes. Cytochrome b561 plays a fundamental role in the physiology of all neuroendocrine tissues, and the results presented here show that Xenopus cytochrome b561 mRNA is specific to neuroendocrine tissues and is developmentally regulated at the gastrula stage.