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Sharma, Rita (2006): Phosphorylation of chloroplast preproteins and the characterisation of nucleoside diphosphate kinase 2. Dissertation, LMU München: Faculty of Biology
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Abstract

The nuclear-encoded preproteins destined for chloroplasts are phosphorylated at a serine or threonine residue in their transit peptides by a cytosolic protein kinase. A phosphorylation motif was proposed for these sites of phosphorylation. In the present study, the sites of phosporylation were investigated for the proteins APC1, HCF136, CAO of Arabidopsis thaliana and pea LHCP. In case of APC1 and LHCP, it was found that more than one site of phosphorylation exist in their transit peptides. For HCF136, the actual site of phosphorylation differred from the site determined by the proposed phosphorylation motif, whereas the transit peptide of CAO was not found to be phosphorylated at all. Further, it was tried to find the significance of this phosphorylation on the import process, in vivo. The second part of this work involved the study of the isoform of the ubiquitous housekeeping enzyme, the Nucleoside diphosphate kinase (NDPK2). The NDPK2 from pea had been earlier reported to have two forms in the chloroplast. The detailed biochemical characterisation of these forms was taken up here. No difference between their import behaviours and localisations within chloroplasts could be determined. They were both found to be in the stroma, associated with thylakoids and nucleoids. In recent years, a number of groups have reported the presence of the NDPK2 of Arabidopsis thaliana to be in the nucleus and cytosol which contradicts with earlier studies conducted with pea, spinach and Brassica campestris where this isoform was found to be in chloroplasts. The present work shows that NDPK2 from Arabidopsis thaliana is also localised within the chloroplasts.