The head structure of a higher plant V-type H+-ATPase is not always a hexamer but also a pentamer

Pentameric head structures of the V-type H⁺–ATPase ofMesembryanthemum crystallinum L. were demonstrated in additionto hexameric head structures by rotational image analysis andmolecular projections of negatively stained H⁺–ATPaseheads. This observation, at least partially, is in contrastto the standard model of the V-type H⁺–ATPase predictingsolely a hexameric head structure with three A and three B subunitsin analogy to the F-type ATPases. With one A or B subunit missingtwo A or two B subunits would be adjacent to each other in thepentameric ATPase head. By chemical cross-linking of H ⁺–ATPasesubunits a crosslinking product exclusively consisting of Bsubunits, in addition to a cross-linking product consistingof subunits A and B was detected. Thus, the pentameric headsmight lack one A subunit, although the lack of one B subunitcan not be totally ruled out. We assume that the hexameric headstructure is the catalytically active configuration while thepentameric head structure may be a relatively stable intermediateof turnover. Key words: V-type H⁺–ATPase, protein structure, electron microscopy, tonoplast, Mesembryanthemum crystallinum L