Processing and activation of chloroplast l-myo-inositol 1-phosphate synthase from Oryza sativa requires signals from both light and salt

Abstract

Two forms of enzymatically active l-myo-inositol 1-phosphate synthase(s) have been detected in the chloroplasts of Oryza sativa L. The two forms have been identified as comprising of ~80 and ~60 kDa subunits. The ~80 kDa subunit is the predominant species in the dark grown etioplasts while the light/dark grown chloroplasts accumulate the ~60 kDa subunit. The larger subunit is mostly membrane bound and the smaller one accumulates in the stromal fraction. Purified ~80 kDa subunit is proteolytically cleaved to the ~60 kDa subunit by chloroplastic supernatant immunodepleted of the synthase protein. Further, in seedlings of salt tolerant varieties of Oryza grown under light/dark environment in presence of 100 mM NaCl, the ~60 kDa subunit is phosphorylated in a Ca⁺² dependent manner, commensurate with increased enzymatic activity. It appears that a light and salt mediated interplay of protease(s) and kinase(s) system regulates the processing and activation of the chloroplast inositol synthase(s) in higher plants.