Review of previous work

Background to plant Ca2+-dependent, calmodulin-independent protein kinases (CPKs)

A brief review of Ca2+-regulated protein kinases in plants is useful because it places in context the smaller group of CPKs we are concerned with here. It also demonstrates that there is now considerable descriptive information available for CPKs, but that this precedes a clear understanding of their function(s) in plant cells.

By the beginning of the 1980s, researchers working on signal transduction in animals had described two main types of protein kinases, whose activity was controlled by cAMP and Ca2+/calmodulin (Cohen, 1982). Therefore the first papers describing Ca2+-regulated protein kinases from plants looked with particular interest for a role for calmodulin (Hetherington & Trewavas, 1982; Polya & Davies, 1982). During this time the evidence was accumulating for protein kinase C in animal cells (Takai et al., 1977, 1979; Nishizuka, 1984). There then followed several papers in which the focus of interest was the role of phospholipid and diacylglycerol in controlling Ca2+-dependent protein kinase activity in plant cells (Schäfer et al., 1985; Muto & Shimogawara, 1985; Elliott & Skinner, 1986). However, there were anomalies in the work so far described; for example, the relatively high concentrations of calmodulin needed for activation effects, and interaction of calmodulin with the histone substrate (see Polya et al., 1990, for discussion); and the general absence of phorbol ester effects on putative protein kinase C activities in plant cells (see Hetherington, Battey & Millner, 1990).