Characterization of plant heterotrimeric G proteins and their interacting partners

This thesis focuses on determining the functional role of plant G proteins through identifying their interacting partners and studying the activities of plant G proteins expressed in S. cerevisiae. The G protein beta subunit was isolated from Pisum sativum cDNA library. Pea Gbeta was shown to be encoded by a single-copy gene and its transcript levels were analyzed in different plant organs and under different growth conditions. The growth-related activities of plant G protein subunits expressed in S. cerevisiae were studied. Pea Gbeta expression was shown to downregulate yeast colony formation. Arabidopsis Gbeta expression restored growth in pheromone-treated or Gbeta subunit-overexpressing yeast cells.; Using yeast two-hybrid library screens, Arabidopsis Galpha subunit-interacting partners were identified and interactions were confirmed in vitro. Five Arabidopsis Galpha subunit-interacting proteins have been isolated and characterized: (1) a novel CTR1/Raf-like protein CLK, (2) a pirin protein (a possible transcriptional regulator), (3) a prephenate dehydratase-like protein, (4) a novel protein which may be involved in plant defense processes and (5) a plant homedomain protein (PHD-finger protein). The CLK was determined to be a true protein kinase interacting preferentially with an inactive form of alpha subunit. By screening a T-DNA insertion library, a pirin gene insertion mutant was identified. It was established that a homozygous pirin insertion leads to lethality and embryos carrying a homozygous insertion in the pirin gene are arrested in their morphological development at a globular stage of embryogenesis.