The 32 kDa tonoplast polypeptide Di associated with the V0‐type H+‐ATPase of Mesembryanthemum crystallinum L. in the CAM state: A proteolytically processed subunit B?

Abstract

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 alignment of its N-terminal protein sequence with protein sequences of recently cloned higher plant V-ATPase B-subunits indicates that D_i may be derived from subunit B by proteolytic removal of a protein fragment of about 20 kDa from its N-terminus. Furthermore, an antiserum directed against D_i cross-reacts with subunit B from Nicotiana tabacum. It 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.