THE ATP SYNTHASE OF HALOBACTERIUM SALINARIUM (HALOBIUM) IS AN ARCHAEBACTERIAL TYPE AS REVEALED FROM THE AMINO-ACID-SEQUENCES OF ITS 2 MAJOR SUBUNITS

The head piece of the A-type ATP synthase in an extremely halophilic archaebacterium, namely Halobacterium salinarium (halobium), is composed of two kinds of subunit, α and β, and is associated with ATP-hydrolyzing activity. The genes encoding these subunits with hydrolytic activity have been cloned and sequenced. The putative amino acid sequences of the α and β subunits deduced from the nucleotide sequences of the genomic DNA consist of 585 and 471 residues, respectively. The amino acid sequence of the α subunit of the halobacterial ATPase is 63 and 49% identical to the a subunits of AT-Pases from two other archaebacteria, Methanosarcina barkeri and Sulfolobus acidocaldarius, respectively. The sequence of the β subunit is 66 and 55% identical to the β subunits from these respective organisms. The homology between the α and β subunits is around 30%. In contrast, the sequences of the halobacterial ATPase is less than 30% identical to F1 ATPase when any combination of subunits is considered. However, they are greater than 50% identical to a eukaryotic vacuolar ATPase when α and a, β and b combinations are considered. These data fully confirm the first demonstration of this kind of relationship which was achieved by immunoblotting with an antibody raised against the halobacterial ATPase. We concluded that the archaebacterial ATP synthase is an A-type and not an F-type ATPase. This classification is also demonstrated by a "rooted" phylogenetic tree where halobacteria locate close to other archaebacteria and eukaryotes and distant from eubacteria. © 1991.