The six par (partitioning defective) genes function to establish and maintain asymmetry in the early embryo. The
par-4 gene is the most recently cloned of the six, and it encodes a putative serine-threonine kinase with similarity to Xenopus and human kinases (J. Watts and D. G. Morton, manuscript in preparation). The phenotypes caused by
par-4 mutations include smaller oocytes, abnormal cytoplasmic organization in the newly fertilized zygote, less pronounced pseudocleavage, synchronous cell divisions at the two and four-cell stages, mislocalization of P granules, and arrest as a differentiated mass of cells lacking intestine. To understand the role of PAR-4 in cytoplasmic organization, we are taking two approaches to identify potential substrates of the putative kinase and other proteins with which it interacts. First, we are using full-length, truncated and mutant forms of PAR-4 in a two-hybrid screen. Second, we plan to screen a cDNA expression library for phosphorylation by an active PAR-4 kinase. Candidates identified using these two approaches will be further screened using RNA interference.