Structure of the Higher Plant Light Harvesting Complex I:  In Vivo Characterization and Structural Interdependence of the Lhca Proteins

Abstract

We have investigated the structure of the higher plant light harvesting complex of photosystem I (LHCI) by analyzing PSI-LHCI particles isolated from a set of Arabidopsis plant lines, each lacking a specific Lhca (Lhca1-4) polypeptide. Functional antenna size measurements support the recent finding that there are four Lhca proteins per PSI in the crystal structure [Ben-Shem, A., Frolow, F., and Nelson, N. (2003) Nature 426, 630-635]. According to HPLC analyses the number of pigment molecules bound within the LHCI is higher than expected from reconstitution studies or analyses of isolated native LHCI. Comparison of the spectra of the particles from the different lines reveals chlorophyll absorption bands peaking at 696, 688, 665, and 655 nm that are not present in isolated PSI or LHCI. These bands presumably originate from "gap" or "linker" pigments that are cooperatively coordinated by the Lhca and/or PSI proteins, which we have tentatively localized in the PSI-LHCI complex.

Characterization of Low-Energy Chlorophylls in the PSI-LHCI Supercomplex from Chlamydomonas reinhardtii. A Site-Selective Fluorescence Study

“…30,[39][40][41][42][43] A sharp band at 696 nm also was observed in PSI-LHCI from Arabidospsis and attributed to pigments in the interfaces between LHCI subunits or between LHCI and PSI. 44 We may speculate that the second pool, at 701 nm, correspond to one to two red Chls located close to the RC as reported for C. reinhardtii. 28,29 The sharp character of the 697-and 701-nm bands may indicate that the mechanism of their red shift is different from the C700 pool: for example that they originate from monomeric Chls which are tuned by the protein environment.…”

The Arabidopsis nox Mutant Lacking Carotene Hydroxylase Activity Reveals a Critical Role for Xanthophylls in Photosystem I Biogenesis

“…Since xanthophyll depletion in nox plants limits the total amount of LHC, this might enhance photooxidative stress, consistent with the increased PSI core degradation in nox mutant (Figure 7). Nevertheless, several Arabidopsis mutants strongly depleted in LHCI have been described (Klimmek et al, 2005;Havaux et al, 2007), and all of them maintain a functional PSI core complex. Alternatively, the steady state level of PSI and PSII core complexes could be limited by chlorophyll availability; indeed, a coregulation of chlorophyll and carotenoid biosynthesis has been reported (Härtel et al, 1997).…”

Lhca5 interaction with plant photosystem I

“…This band contains cross-linked dimers of Lhca2/a3 but also Lhca5 cross-linked either to Lhca2, Lhca3, or another Lhca5. We favour a Lhca2-Lhca5 dimer as PSI particles prepared from plants genetically depleted in Lhca2 contain less than 5% of wt-levels of Lhca5 [8]. The combination of both findings strongly indicate that Lhca5 interacts with PSI via Lhca2 at the Lhca2/a3-site of PSI.…”

“…Four of the Lhca-type proteins (Lhca1-Lhca4) assemble at subtypespecific positions with one copy each as an external antenna (LHCI) with the PSI core on the PsaF-site of the complex [6]. The depletion of one protein-type affects the association of the others [7,8] and this interdependence was shown to be strongest between Lhca1/a4 and Lhca2/a3. While heterodimers of Lhca1/a4 have been isolated from native PSI-LHCI preparations [9,10] and reconstituted in vitro [11] this has not been achieved for Lhca2/a3, yet.…”