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The halo‐opsin gene. II. Sequence, primary structure of halorhodopsin and comparison with bacteriorhodopsin
Author(s) -
Blanck A.,
Oesterhelt D.
Publication year - 1987
Publication title -
the embo journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 7.484
H-Index - 392
eISSN - 1460-2075
pISSN - 0261-4189
DOI - 10.1002/j.1460-2075.1987.tb04749.x
Subject(s) - halorhodopsin , biology , gene , ribosomal rna , protein primary structure , genetics , microbiology and biotechnology , bacteriorhodopsin , peptide sequence , membrane
The gene for the protein moiety of the light‐driven chloride pump halorhodopsin (HR), hop gene, was sequenced and the primary structure of the protein derived thereof. The gene has a GC content of 67% and codes for 274 amino acids. A promoter structure, resembling that of the halobacterial 16S rRNA genes, is present and both a terminating stem and a loop sequence is found downstream of the TGA stop codon. A ribosomal binding site is located within the translated region. The HR protein moiety is processed at the amino terminus, as well as the carboxy terminus, yielding a dominant species of calculated M r 26 961. Seven transmembrane helical parts of the protein are defined by hydropathy and acrophilicity calculations. Comparision with the bacteriorhodopsin (BR) structure reveals a conservation of 36% of amino acid residues in the transmembrane part and 19% in the connecting loops at both surfaces. The most conspicuous conserved amino acids are the retinal‐binding Lys residue, four Trp residues (eventually interacting with retinal), two Asp residues (providing possibly the negative charge environment of retinal) and three Pro residues of unknown function. No significant homology with the opsins of eucaryotes was found. Helical wheel analysis shows that HR is an inside‐out protein with the majority of conserved amino acid residues inside the circle of the seven transmembrane helices. It is postulated that the intrahelical spaces, which could be gated by the retinal moiety, are the physical entities for translocation of protons in BR and chloride ions in HR. Retinal, by its cis−trans isomerization, serves as a switch connecting the ion‐specific binding sites in both proteins.