Receptor protein kinase acting as an integrator for intracellular calcium and heterotrimeric G protein signaling with extracellular signals during early zygote development. Involved in modulating the asymmetric division of zygote and the cell fate determination of its daughter cells. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Part of an in vivo complex containing ZAR1, CAM1 and GB1. Interacts (via CaMBD domain) with calmodulins CAM1 and CML8 in a calcium independent manner. Interacts (via GBeta-binding domain) with GB1. Expressed at the micropylar nuclei of the embryo sac at eight-nucleate stage before cellularization, but not in chalazal nuclei. After cellularization, expressed in central cell and synergids in mature embryo sac. After fertilization, detected specifically in zygote, endosperm precursor cell, and later in endosperm. In addition, also present in sperm and the vegetative cells in both pollen and pollen tube. Expressed at high level in the egg cell, the central cell, the endosperm, and at low level in the zygote. The CaMBD domain (358-373) is required for calmodulin binding. The GBeta-binding domain (577-584) is required for GB1 binding. The protein kinase domain is predicted to be catalytically inactive. Autophosphorylated. The kinase activity is activated by the binding of CAM1 and/or GB1. Embryo lethal when homozygous. Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.