THE 32 KDA TONOPLAST POLYPEPTIDE D-I ASSOCIATED WITH THE V-TYPE H-ATPASE OF MESEMBRYANTHEMUM-CRYSTALLINUM L IN THE CAM STATE - A PROTEOLYTICALLY PROCESSED SUBUNIT-B()

In the facultative halophyte Mesembryanthemum crystallinum, the salt- or age-induced transition to crassulacean acid metabolism (CAM) leads to the occurrence of a tonoplast-bound 32 kDa polypeptide (D-i). The a lignment of its N-terminal protein sequence with protein sequences of recently cloned higher plant V-ATPase B-subunits indicates that D-i ma y be derived from subunit B by proteolytic removal of a protein fragme nt of about 20 kDa from its N-terminus. Furthermore, an antiserum dire cted against D-i cross-reacts with subunit B from Nicotiana tabacum. I t inhibits both proton pumping and ATP hydrolysis of the holoenzyme in M. crystallinum. As D-i remains firmly attached to the holoenzyme the proteolytic processing may have functional implications.