A soluble protein with a molecular mass of 55 kDa has been purified from etiolated pea stem mitochondria. The protein exhibits a Mg2+-requiring PPiase activity, with an optimum at pH 9.0, which is not stimulated by monovalent cations, butinhibited by F2,Ca2+, aminomethylene diphosphate and imidodiphosphate. The protein does not cross-react with polyclonal antibodies raised against vacuolar, mitochondrial or soluble PPiases, respectively. Conversely, it cross-reacts with an antibody for thea/b-subunit of the ATP synthase from beef heart mitochondria. The purified protein has been analyzed by MALDI-TOF
mass spectrometry and the results, covering the 30% of assigned sequence, indicate that it corresponds to theb-subunit of the ATP synthase of pea mitochondria. It is suggested that this enzymatic protein may perform a dual function as soluble PPiase or as subunit of the more complex ATP synthase.
