The role of PSI-G and PSI-K of higher plants in the interaction between light harvesting complex I and the photosystem I reaction center core.

Abstract

PSI-G and PSI-K are subunits of photosystem I (PSI). PSI-G is unique to higher plants whereas PSI-K is also found in PSI of cyanobacteria. The function of PSI-G and PSI-K was characterized in Arabidopsis plants transformed with a psaG or psaK cDNA in antisense orientation and plants without detectable PSI-G or PSI-K protein were identified. There was no obvious visible difference between wild-type plants and plants lacking PSI-G or PSI-K. Steady state electron transport of PSI in vitro was unaffected in the absence of PSI-G or PSI-K. Plants without PSI-G and PSI-K had 50% and 15% less state1-state2 transitions, respectively. This shows that the redistribution of absorbed excitation energy between PSII and PSI is reduced when either of the two polypeptides is missing in PSI. Low-temperature fluorescence emission spectra revealed a 2-nm blue-shift in the long wavelength emission in plants lacking PSI-K and a 1-nm blue-shift in plants devoid of PSI-G suggesting a perturbation of the LHCI antenna in the absence of PSI-G or PSI-K. Furthermore, thylakoids and isolated PSI without PSI-K had 20-30% less Lhca2 and 30-40% less Lhca3 while Lhca1 and Lhca4 were unaffected. In contrast, the absence of PSI-G only resulted in a minor reduction in Lhca proteins. During electrophoresis of pigment-protein complexes under mildly denaturing conditions, all four Lhca subunits were partially dissociated from PSI lacking PSI-K. Surprisingly, the absence of PSI-G resulted in even more loosely bound Lhca proteins. The observed effects demonstrate that PSI-G and PSI-K are not strictly necessary for attachment of the light harvesting complexes to the core but are important for stable interaction.