Phosphatidylserine-sensitive calcium dependent protein kinase in rice embryo

Abstract

Protein kinase activity in the embryos of rice ( Oryza sativa L. cv. Nipponbare) during seed soaking in water was assessed under various conditions. This activity was increased by the addition of Ca 2+ and phosphatidylserine (PS), but not by EGTA or Ca 2+ alone. The potent phorbol ester did not augment this increase. The M r of the protein kinases were 55 000 and 56 000. The phosphorylated proteins were separated by two-dimensional gel electrophoresis. Two proteins with M r 40 000 and isoelectric points of 8.9 and 7.6 were significantly phosphorylated with Ca 2+ and PS in vitro. They were first observed after 2 days of soaking, accumulated thereafter and disappeared after 4 days. When the membrane fraction was incubated in Ca 2+ and PS, proteins in the fraction were phosphorylated. Phosphorylation of the proteins with Ca 2+ and PS was inhibited by staurosporine and H-7, but not by W-7 or okadaic acid. The proteins were also found in wheat embryos, but wheat proteins were phosphorylated by Ca 2+ alone. The present data suggest that activation of PS-sensitve Ca 2+ dependent protein kinase associated with membrane and subsequent protein phosphorylation may be involved in regulating the early stages of rice development.