Some of the new proteins that have been identified by Dr. Zeeman and other researchers in the field act as glucan kinases and phosphatases, that is, they place and remove phosphate groups on the starch molecules. Among these proteins are glucan water dikinase (GWD), phosphoglucan water dikinase (PWD) and SEX4. It is thought that GWD and PWD act in concert to place phosphate groups on starch molecules. The highly charged phosphate group may act as a wedge, disrupting the semi-crystalline packing in the starch and allowing degradative enzymes access to the glucose chains and branches. SEX4 then removes the phosphate groups. Although the exact mechanisms of how these proteins coordinate starch metabolism are still unknown, the importance of phosphate groups in the process is now well established. Mutants of all of these proteins result in plants with an excess of undigested starch.
Sequencing analyses have shown that GWD is conserved over many plant taxa, and proteins similar to SEX4 have been found in other plant species, including rice, maize, and tomato. The amylopectins of leaf starches in different plant species have also been found to be decorated with phosphate groups. Studies of A
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American Society of Plant Biologists