CALMODULIN-STIMULATED CALCIUM-PUMPING ATPASES LOCATED AT HIGHER-PLANTINTRACELLULAR MEMBRANES - A SIGNIFICANT DIVERGENCE FROM OTHER EUKARYOTES

The plasma membrane (PM) of all eukaryotes so far investigated contain s a P-type Ca2+-pumping ATPase responsible for maintaining low cytosol ic free calcium concentrations. In animal cells this has been shown to be a type of Ca2+-pump which is directly stimulated by binding the ca lcium-dependent regulator protein calmodulin. These PM Ca2+-pumps have been named 'PM-type' as they appear to be exclusively located at the PM and not in intracellular membrane (IM) fractions. Recent progress o n higher plant cells reveals that they possess calmodulin-stimulated C a2+-pumps of the 'PM-type'. However, these calmodulin-stimulated Ca2+- pumps appear to be located not only at the PM but also in intracellula r membranes, probably the endoplasmic reticulum (ER). The evidence is also convincing that these IM-located Ca2+-pumps are directly stimulat ed by calmodulin (possess a calmodulin-binding region) and are true 'P M-type' Ca2+-pumps. This appears to represent a marked divergence betw een plant and animal cell Ca2+-pumps. Recently, molecular cloning has revealed that plant cells also contain a Ca2+-pump which is not direct ly stimulated by calmodulin and which strongly resembles the mammalian ER/SR type of Ca2+-pump. The significance of these findings for plant cell function is discussed.