The PSI-K Subunit of Photosystem I Is Involved in the Interaction between Light-harvesting Complex I and the Photosystem I Reaction Center Core

Abstract

PSI-K is a subunit of photosystem I. The function of PSI-K was characterized in

Single and Double Knockouts of the Genes for Photosystem I Subunits G, K, and H of Arabidopsis. Effects on Photosystem I Composition, Photosynthetic Electron Flow, and State Transitions

“…Interestingly, the drop in total Chl level was not detected in psag-1.4/psah2-1.4, or psag-1.4/psak-1. The two genotypes lacking PSI-K, psak-1, and psak-1/psag-1.4 showed an increase in the Chl a/b ratio, which is consistent with the increased PSI to PSII ratio monitored in PSI-K cosuppression lines (Jensen et al, 2000). In psag-1.4 plants, the Chl a/b ratio was slightly reduced, indicating either an increased peripheral antenna or more likely, in light of the immunoblot analyses described above, a somewhat lower PSI/PSII ratio.…”

“…This finding is compatible with a role for PSI-G in stabilizing the PSI core. The evolutionarily related PSI-K subunit, on the other hand, does not have such a stabilizing function (Jensen et al, 2000; this study). According to the current working model for the structure and topology of PSI (Scheller et al, 2001), PSI-G is located peripherally, and forms contacts with an Lhca2 homodimer (Jansson et al, 1996).…”

“…The only slight decrease in amounts of Lhca2 seen in psag-1.4 does not support the idea that Lhca2 is exclusively bound to PSI-G. However, the marked drop in levels of Lhca3 in plants lacking PSI-K (Jensen et al, 2000; this study), imply that, in the WT, homodimers of Lhca3 are bound to PSI-K. Three lines of evidence strongly suggest that Lhca2 and Lhca3 homodimers are in physical contact with each other: (a) Lhca2 and Lhca3 can be chemically cross-linked (Jansson et al, 1996), (b) PSI-K-less lines exhibit a secondary loss of Lhca2 (Jensen et al, 2000; this study), and (c) antisense inhibition of either Lhca2 or Lhca3 synthesis results in a concomitant decrease in the levels of both proteins (Ganeteg et al, 2001). Cross-linking experiments provide no support for the idea that Lhca2 and Lhca3 form heterodimers (Jansson et al, 1996), which would otherwise explain this interdependence.…”

“…Such analyses have revealed that PSI-K plays a role in organizing LHCI (Jensen et al, 2000), PSI-N is necessary for the interaction of plastocyanin with PSI (Haldrup et al, 1999), and PSI-H putatively is the attachment site for LHCII during state transitions (Lunde et al, 2000). Loss of PSI-E or -F has far more dramatic consequences on the photoautotrophic growth of Arabidopsis than corresponding mutations in cyanobacteria or algae (Chitnis et al, 1989(Chitnis et al, , 1991Zhao et al, 1993;Xu et al, 1994;Farah et al, 1995): In Arabidopsis, the entire stromal side of PSI is affected by disruption of the psaE1 gene (Varotto et al, 2000; unpublished data in our lab), whereas deficiency of PSI-F results in significantly decreased levels of most PSI polypeptides (Haldrup et al, 2000), making it difficult to distinguish between primary and secondary effects of the loss of these subunits in Arabidopsis.…”

“…To determine whether this lack of severe phenotypic effects is due to redundancy, arising because one or more of the remaining PSI subunits can functionally substitute for the product of the defective gene, PSI-G mutants that were partially deficient in -H or completely lacked -K were tested for photosynthetic electron flow and the organization of PSI was characterized. The consequences of the simultaneous loss of PSI-G and -K were of particular interest because these two proteins are evolutionarily related and have been proposed to play similar roles in the organization of LHCI (Kjaerulff et al, 1993;Jansson et al, 1996;Jensen et al, 2000).…”