A Chlorophyll a/b-binding Protein Homolog That Is Induced by Iron Deficiency Is Associated with Enlarged Photosystem I Units in the Eucaryotic Alga Dunaliella salina

Sharon G Wolf,Tal Varsano,Uri Pick

doi:10.1074/jbc.m511057200

Sharon G Wolf, Tal Varsano + Show 1 more

Open Access

https://doi.org/10.1074/jbc.m511057200

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Abstract

Adaptation of the halotolerant alga Dunaliella salina to iron deprivation involves extensive changes of chloroplast morphology, photosynthetic activities, and induction of a major 45-kDa chloroplast protein termed Tidi. Partial amino acid sequencing of proteolytic peptides suggested that Tidi resembles chlorophyll a/b-binding proteins which compose light-harvesting antenna complexes (LHC) (Varsano, T., Kaftan, D., and Pick, U. (2003) J. Plant Nutr. 26, 2197-2210). Here we show that Tidi shares the highest amino acid sequence similarity with light-harvesting I chlorophyll a/b-binding proteins from higher plants but has an extended proline-rich N-terminal domain. The accumulation of Tidi is reversed by iron supplementation, and its level is inversely correlated with photosystem I (PS-I) reaction center proteins. In native gel electrophoresis, Tidi co-migrates with enlarged PS-I-LHC-I super-complexes. Single particle electron microscopy analysis revealed that PS-I units from iron-deficient cells are larger (31 and 37 nm in diameter) than PS-I units from control cells (22 nm). The 77 K chlorophyll fluorescence emission spectra of isolated complexes suggest that the Tidi-LHC-I antenna are functionally coupled to the reaction centers of PS-I. These findings indicate that Tidi acts as an accessory antenna of PS-I. The enlargement of PS-I antenna in algae and in cyanobacteria under iron deprivation suggests a common limitation that requires rebalancing of the energy distribution between the two photosystems.

Highlights

In the green alga Chlamydomonas reinhardtii, iron deprivation is associated with remodeling of photosystem I (PS-I), resulting in changes in the polypeptide composition of light-harvesting antenna complexes (LHC)-I and in a decreased efficiency of energy transfer between the light harvesting and reaction center (RC) [7, 8]
thylakoid protein of kDa (Tidi) Is Localized in Thylakoid Membranes and Is Resolved into Five Components on Two-dimensional Gels—In a previous study we reported that iron deficiency induces the accumulation of a 45-kDa protein in a thylakoid-enriched preparation from D. salina
Based on a partial amino acid sequence homology of two proteolytic fragments, obtained by Edman degradation, we proposed that the protein is a chlorophyll a/b-binding protein [12]

Summary

Introduction

Complexes 1 and 2 from iron-deficient and from control cells have similar Chl fluorescence emission spectra, with two peaks at 685 and 696 nm, which correspond to PS-II RC (Fig. 8, top) [27, 28]. Suggest that the detergent-induced changes in Chl fluorescence emission reflect reversible uncoupling of energy transfer from LHC-I antenna from PS-I RCs. This interpretation is consistent with a previous analysis of isolated PS-I units from C. reinhardtii, which demonstrated detergent-induced dissociation of LHC-1 from the PS-I core, with corresponding 77 K fluorescence emissions at 675 and 715 nm, respectively [33], and with similar observations in cyanobacteria [5].

Results

Conclusion