THE STOMATAL PHOSPHOENOLPYRUVATE CARBOXYLASE - A POTENTIAL TARGET FORSELECTIVE PROTEOLYSIS DURING STOMATAL CLOSURE

The activity of the stomatal key enzyme phosphoenolpyruvate carboxylas e (PEPCase, EC 4.1.1.31) is known to be decreased during ABA-induced s tomatal closure, in parallel the level of the second messenger inosito l 1,4,5 trisphosphate (ins(1,4,5)P-3) is increased. Since ins(1,4,5)P- 3 itself induces stomatal closure, the effect of the second messenger on protein-selective degradation of PEPCase and the specificity for PE PCase guard cell isoforms was investigated. PEPCase was immunoprecipit ated from ins(1,4,5)P-3 treated epidermal crude extracts. Analysis of the precipitates gave hints to an ins(1,4,5)P-3-induced selective ubiq uitin-dependent proteolysis preferentially of guard cell specific PEPC ase isoforms (110,112,116 kDa). The role of the second messenger ins(1 ,4,5)P-3 as a possible mediator between signal-mediated cascades and t he ubiquitin-specific PEPCase degradation is discussed on the basis of PEPCase functioning as a target for selected proteolysis during stoma tal closure. Moreover, the diurnal rhythmicity of guard cell protein t urnover was analysed, especially with respect to ubiquitin-dependent p rocesses.