PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Site-specific proteolytic processing of LHCA3 is a key event in the remodeling of light-harvesting antenna under iron-deficiency in Chlamydomonas reinhardtii. Michael Hippler^{*,1, 2}, Blandine Rimbault², Einar Stauber², ¹ Department of Biology, Philadelphia, PA, USA² Plant Physiology, Jena, Germany

ABSTRACT- Iron deficiency is a widespread disease in plants and animals. A recent study (1) probed the causes of the structural and functional changes of the photosynthetic machinery of Chlamydomonas reinhardtii and elucidated molecular mechanisms that trigger changes of a eukaryotic photosynthetic apparatus in response to iron deficiency. A central feature of this response, the uncoupling of the LHCI antenna from photosystem I, precedes the onset of chlorosis and appears to be mediated through the PSI-K subunit of photosystem I (PSI). We further investigated the changes to the composition of the LHCI complex in thylakoids isolated from cells grown in iron-sufficient and iron-deficient conditions. In particular, an amino-terminally processed form of Lhca3 that results from a proteolytic processing event was identified for Fe (-) conditions using a combination of two dimensional gel electrophoresis, SDS-PAGE, immunoblotting and tandem mass spectrometry (MS/MS). Analysis of sucrose density gradient separated thylakoids demonstrated that the processed form of Lhca3 remains bound to the PSI core under Fe (-). For in-depth quantification arginine auxotrophic C. reinhardtii cells were grown in the presence of either isotopically labeled or unlabeled arginine ([13C6]Arg ). Thus allowing a comparative quantification of labeled and unlabeled sister peptides originated from both conditions by MS/MS. We find that proteolytic processing is unique to Lhca3 and that individual Lhca proteins are affected to varying degrees by iron deficiency. Proetolytic processing of Lhca3 was found under photoheterotrophic as well as under photoautotrophic conditions. We suggest that the N-terminal processing of Lhca3 induces a conformational change that triggers (i) the remodeling of LHCI and (ii) thereby decreases the efficiency of excitation energy transfer from the antenna to the PSI reaction center. 1. Moseley, J.L., Allinger, T, et al. (2002) EMBO J, 21, 6709-6720.

KEY WORDS: Iron-deficiency, Proteomics, Adaptation, Chlamydomonas reinhardtii