Plant and fungal calmodulin: Ca 2+-dependent regulation of plant NAD kinase

Abstract

Although little is known about the role(s) of second messengers, including free Ca 2+, in plant cells there has been increasing evidence for a role for Ca 2+ in metabolic regulation in plants. The recent demonstration that the Ca 2+-binding protein, calmodulin exists in extracts of higher plants and basidiomycete fungi provides a basis for understanding Ca 2+-dependent metabolic regulation in plant cells. In this review we summarize the similarities and differences of plant, fungal and mammalian calmodulin. We also discuss the known in , vitro functions of calmodulin in higher plants. A Ca 2+-calmodulin-dependent NAD kinase has been purified to homogeneity from extracts of pea seedlings and shown to be absolutely dependent upon calmodulin and μM levels of free Ca 2+ for activity. The available evidence suggest that this Ca 2+-calmodulin-dependent NAD kinase is the major form of plant NAD kinase and that this regulatory enzyme is localized in the chloroplast. A model is presented which predicts that the rate of photosynthesis is regulated by a receptor-mediated change in the level of chloroplastic free Ca 2+ upon illumination. Free Ca 2+, acting as a second messenger, forms a Ca 2+-calmodulin complex thus converting calmodulin to its active conformation. This Ca 2+-calmodulin complex then activates chloroplastic NAD kinase resulting in an increased NADP NAD ratio.