Pigment Binding, Fluorescence Properties, and Oligomerization Behavior of Lhca5, a Novel Light-harvesting Protein

Volkmar H.R. Schmid,Stefanie Storf,Stefan Jansson

doi:10.1074/jbc.m411248200

Volkmar H.R. Schmid, Stefanie Storf + Show 1 more

Open Access

https://doi.org/10.1074/jbc.m411248200

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Abstract

A new potential light-harvesting protein, named Lhca5, was recently detected in higher plants. Because of the low amount of Lhca5 in thylakoid membranes, the isolation of a native Lhca5 pigment-protein complex has not been achieved to date. Therefore, we used in vitro reconstitution to analyze whether Lhca5 binds pigments and is actually an additional light-harvesting protein. By this approach we could demonstrate that Lhca5 binds pigments in a unique stoichiometry. Analyses of pigment requirements for light-harvesting complex formation by Lhca5 revealed that chlorophyll b is the only indispensable pigment. Fluorescence measurements showed that ligated chlorophylls and carotenoids are arranged in a way that allows directed energy transfer within the light-harvesting complex. Reconstitutions of Lhca5 together with other Lhca proteins resulted in the formation of heterodimers with Lhca1. This result demonstrates that Lhca5 is indeed a protein belonging to the light-harvesting antenna of photosystem I. The properties of Lhca5 are compared with those of previously characterized Lhca proteins, and the consequences of an additional Lhca protein for the composition of the light-harvesting antenna of photosystem I are discussed in view of the recently published photosystem I structure of the pea.

Highlights

Photosystem I (PSI)1 of higher plants is a multi-protein complex located in the thylakoid membrane and can be separated into two moieties, namely the core complex containing the primary donor (P700) that performs the charge separation and the external antenna, the so-called light-harvesting complex I (LHCI)
The recently published crystal structure of pea PSI showed at a resolution of 3.4 Å the presence of four pigmented Lhca proteins attached to one side of the PSI core complex [1]
The apoprotein corresponding to the mature form of Lhca5 of A. thaliana was expressed in E. coli and isolated as inclusion bodies

Summary

Introduction

Photosystem I (PSI) of higher plants is a multi-protein complex located in the thylakoid membrane and can be separated into two moieties, namely the core complex containing the primary donor (P700) that performs the charge separation and the external antenna, the so-called light-harvesting complex I (LHCI). All LHCs consist of a protein backbone that coordinates pigments not covalently in a way that allows directed energy transfer to the reaction center chlorophylls (Chl). In this way, LHCs increase the quantum yield of photosynthesis. Isolation of a pigment-protein complex of Lhca has not yet been achieved, which is most probably due to the small amount of this LHC in PSI and LHCI preparations compared with that of other Lhca proteins. Pigment analyses of native LHCIs (e.g. 5, 19, 21–23) and reconstituted Lhca1–Lhca (r-Lhca1–r-Lhca4) [5, 7, 19, 21, 24] revealed that all four Lhca proteins coordinate Chl a, Chl b, This paper is available on line at http://www.jbc.org. Lhca and Lhca show fluorescence in the 680 – 690-nm range, comparable with that of the LHCs of PSII (5, 7, 18, 19, 24 –26)

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