Protein interactions in signaling networks in Arabidopsis

Sammanfattning: Protein networks drive numerous crucial processes in cells. One important component in their coordination is protein-protein interactions. Mitogen activated protein kinase (MAPK) signaling substrates and endoplasmic reticulum (ER) proteins in Arabidopsis were investigated with respect to their respective interactions. Novel MAPK substrates were screened for and tested for MAPK specific phosphorylation. One of the identified substrates, MAP kinase substrate 80 (MKS80) was characterized to its effect on stomatal patterning (Paper I). Paper II concerns an interacting protein to MKS80, alpha glucan phosphorylase 2 (PHS2). PHS2 was subcellularly localized to the cytoplasm and at least in one instance to the nucleus. Three novel phenotypes of knock-out mutants are described: lesion formation in leave blades upon low light treatment, reduced hypocotyl extension upon sucrose treatment under imbibed conditions and early senescence in a detached leave assay. The underlying mechanisms in this protein network remain to be elucidated. Studies presented in Paper III aimed to characterize MKS80 such as its subcellular localization (to the cytoplasm and nucleus). Gene expression was found to be strongest in senescent tissues using a reporter gene assay. Exploratory investigations on ER proteins as described in part B resulted in the development of a method to extract and separate ER proteins (2 dimensional polyacrylamide gel electrophoresis coupled to mass spectrometry, 2D BN/SDS PAGE) with the aim to detect protein complexes. The chaperone calreticulin was of special interest for further investigations and specific antibodies to two of the three Arabidopsis paralogues were tested. This opens possibilities for future approaches to complement 2D BN/SDS PAGE with. For example areas containing the calreticulin paralogue of interest can be identified specifically or co-immunoprecipitation for identification of protein complexes containing the protein of interest can be performed. Future efforts could reveal paralogue specific protein-protein interactions and lead to a better understanding of protein-dynamics in the ER.