A Calvin cycle multiprotein complex including phosphoribulokinase (PRK), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and a small protein, CP12, has previously been identified. In this article, we have studied this complex in leaves and have shown that dissociation and reassociation of the PRK/GAPDH/CP12 complex occurs in a time frame of minutes, allowing for rapid regulation of enzyme activity. Furthermore, we have shown that the extent of formation and dissociation of the PRK/GAPDH/CP12 complex correlates with the quantity of light. These data provide evidence linking the status of this complex with the rapid and subtle regulation of GAPDH and PRK activities in response to fluctuations in light availability. We have also demonstrated that dissociation of this complex depends on electron transport chain activity and that the major factor involved in the dissociation of the pea complex was thioredoxin f . We show here that both PRK and GAPDH are present in the reduced form in leaves in the dark, but are inactive, demonstrating the role of the PRK/GAPDH/CP12 complex in deactivating these enzymes in response to reductions in light intensity. Based on our data, we propose a model for thioredoxin f -mediated activation of PRK and GAPDH by two mechanisms: directly through reduction of disulfide bonds within these enzymes and indirectly by mediating the breakdown of the complex in response to changes in light intensity.

During mating, budding yeast cells reorient growth toward the highest concentration of pheromone. Bni1p, a formin homologue, is required for this polarized growth by facilitating cortical actin cable assembly. Fus3p, a pheromone-activated MAP kinase, is required for pheromone signaling and cell fusion. We show that Fus3p phosphorylates Bni1p in vitro, and phosphorylation of Bni1p in vivo during the pheromone response is dependent on Fus3p. fus3 mutants exhibited multiple phenotypes similar to bni1 mutants, including defects in actin and cell polarization, as well as Kar9p and cytoplasmic microtubule localization. Disruption of the interaction between Fus3p and the receptor-associated Gα subunit caused similar mutant phenotypes. After pheromone treatment, Bni1p-GFP and Spa2p failed to localize to the cortex of fus3 mutants, and cell wall growth became completely unpolarized. Bni1p overexpression suppressed the actin assembly, cell polarization, and cell fusion defects. These data suggest a model wherein activated Fus3p is recruited back to the cortex, where it activates Bni1p to promote polarization and cell fusion.

The mating response of the budding yeast Saccharomyces cerevisiae is mediated by a prototypical heterotrimeric GTP-binding protein (G protein) and mitogen-activated protein kinase (MAPK) cascade. Although signal transmission by such pathways has been modeled in detail, postreceptor down-regulation is less well understood. The pheromone-responsive G protein α subunit (Gα) of yeast down-regulates the mating signal, but its targets are unknown. We have found that Gα binds directly to the mating-specific MAPK in yeast cells responding to pheromone. This interaction contributes both to modulation of the mating signal and to the chemotropic response, and it demonstrates direct communication between the top and bottom of a Gα-MAPK pathway.

Summary: Mechanistic understanding of the costs and benefits of photoacclimation requires knowledge of how photophysiology is affected by changes in the molecular structure of the chloroplast. We tested the hypothesis that changes in the light dependencies of photosynthesis, nonphotochemical quenching and PSII photoinactivation arises from changes in the abundances of chloroplast proteins in Emiliania huxleyi strain CCMP 1516 grown at 30 (Low Light; LL) and 1000 (High Light; HL) μmol photons m⁻² s⁻¹ photon flux densities. Carbon‐specific light‐saturated gross photosynthesis rates were not significantly different between cells acclimated to LL and HL. Acclimation to LL benefited cells by increasing biomass‐specific light absorption and gross photosynthesis rates under low light, whereas acclimation to HL benefited cells by reducing the rate of photoinactivation of PSII under high light. Differences in the relative abundances of proteins assigned to light‐harvesting (Lhcf), photoprotection (LI818‐like), and the photosystem II (PSII) core complex accompanied differences in photophysiology: specifically, Lhcf:PSII was greater under LL, whereas LI818:PSII was greater in HL. Thus, photoacclimation in E. huxleyi involved a trade‐off amongst the characteristics of light absorption and photoprotection, which could be attributed to changes in the abundance and composition of proteins in the light‐harvesting antenna of PSII.