Calcium-induced protein phosphorylation and changes in levels of calmodulin and calreticulin in maize sperm cells

Abstract

To examine possible calcium (Ca2+)-mediated prefertilization events in male gametes of higher plants, we studied protein phosphorylation and the Ca2+-binding proteins, calmodulin and calreticulin, in sperm cells isolated from maize (Zea mays L.) pollen in the presence and absence of Ca2+. Using immunoblotting, we detected calmodulin and calreticulin and Ca2+-induced variations. Exposure of sperm cells to 1 mM Ca2+ for 1 h increased calmodulin content by 136% compared with the control. Ca2+ had little effect on calreticulin at 1 h, but induced a 34% increase after 3 h. Phosphorylation of proteins was low in 1 h-control and Ca2+-treated cells. However, a 13-fold increase in phosphorylation of a 18-kDa protein was found at 12 h in the presence of Ca2+. Ca2+-induced changes in calmodulin, calreticulin and protein phosphorylation observed in maize sperm cells may reflect prefertilization changes in vivo that facilitate sperm cell fusion with egg and central cells.