THE PRIMARY STRUCTURE OF HALOCYANIN, AN ARCHAEAL BLUE COPPER PROTEIN,PREDICTS A LIPID ANCHOR FOR MEMBRANE FIXATION

Halocyanin, a small blue copper protein, was isolated from the haloalk aliphilic archaeon Natronobacterium pharaonis. The NH2 terminus was no t accessible to Edman degradation. About 70% of the amino acid sequenc e was determined by protein sequence analysis. The sequence informatio n of two peptides was used for cloning and sequencing the halocyanin g ene (hcy). The open reading frame codes for 489 base pairs, which acco unt for a protein with 163 amino acids and a molecular mass of 17,223 Da. The discrepancy between this value and the molecular mass of 15,45 6 +/- 1.5 Da for the copper-free protein determined by electrospray ma ss spectrometry can be explained by a post-translational processing of the gene product. The NH2-terminal sequence of the open reading frame contains a motif that is characteristic for prokaryotic lipoproteins. Assuming a similar processing for halocyanin, Cys at position 25 of t he primary transcript would be modified by a diphytanyl (glycerol)diet her. Subsequently, the precursor is cleaved by a signal peptidase II-l ike protease and then acetylated at its NH2-terminal alpha-amino group . These modifications would yield a protein with a calculated molecula r mass of 15,456 Ha. A comparison of the primary structure of halocyan in with a number of other blue copper proteins places it into the plas tocyanin-related group.