Calcium-regulated protein kinases of plants

The importance of calcium ions in coupling physiological responses to external and developmental signals in plants has been well documented. Recently, Plieth and Trewavas (Plant Physiology, 2002, 129: 786-796) have shown that gravistimulation, too, elicits changes in the concentration of cytoplasmic Ca2+ in Arabidopsis plants. Cytoplasmic calcium brings about responses by interacting with target proteins, many of which contain EF-hand calcium-binding motifs. In plants there are at least five classes of protein kinases, all of which are in the CDPK/SnRK family, that either contain EF-hands within their structure or interact with proteins that contain EF-hands. Calcium-dependent protein kinases (CDPKs) and calcium and calmodulin-activated protein kinases (CCaMKs) both contain EF hands in their C-terminal domains and are activated by the binding of calcium. SnRK3s (Group 3 of the SNF-1 related kinases) bind to proteins that contain three EF hands, and some are activated by calcium. Members of two other protein kinase classes, plant calmodulin-dependent protein kinase (CaMK) and CDPK-related kinase (CRK), bind to calmodulin, but it remains to be seen whether the activity of these enzymes is regulated by calcium/calmodulin. This paper will review what is known about the structure and the regulation of these protein kinases and address the question of why there is such a plethora of calcium-regulated kinases in plants.