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In a process termed secondary endosymbiosis a red alga was established as an endosymbiont. This led to the evolution of the complex plastids found in cryptophytes, haptophytes, heterokontophytes and apicomplexans. The genome of this former red alga was drastically reduced and mostly transferred into the host nucleus. Thus, most plastidal proteins are now encoded in the host nucleus and have to be imported from the cytosol into the plastid. Appropriate transport mechanisms had to be evolved. The aim of this work was the identification of new components of the third plastid membrane. It evolved from the outer chloroplast envelope of primary plastids. This membrane is distinct from other plastidal membranes due to the fact that it presumably contains β-barrel proteins which are characteristic of outer membranes of gram-negative bacteria, mitochondria and chloroplasts. With different algorithms putative plastidal β-barrel proteins were predicted and localized as fusion proteins in P. tricornutum. Out of 23 proteins, four were localized to the plasma membrane. Four other proteins were localized to the cytosol. Five proteins were localized to the ER. Additionally five proteins were localized to various compartments outside the plastid and five proteins were localized to the plastid. There were no new proteins of the third plastidal membrane identified. The used prediction algorithms are not suited to analyze a eukaryotic genome for β-barrel proteins. Moreover, the quality of the predicted gene models of the database complicated the analysis.
In the second part of this work the subcompartmentalization of the ER of P. tricornutum was analyzed. Based on the classification of the ER into a hostER, nuclear envelope and cER the functions of these two subcompartments were tested. Based on the assumption that hER and cER are exposed to different physiological conditions during night and day, functions as protein quality control and protein folding might be restricted to the hER. Therefore, factors of the UPR were studied. Surprisingly, IRE1 and PERK were identified in heterokontophytes and haptophytes. Localization studies of these UPR factors showed that these are indeed mainly found in the hER and only in minor concentrations in the nuclear envelope and cER. hDer1-2 which takes part in protein degradation shows an equal distribution over the complete ER, while transporter proteins like Tpt1 are mainly found in the cER and nuclear envelope. The absence of the UPR factors in the cER can be explained by different physiological conditions during night and day inside of these two subcompartments. Additionally, the presence of PERK and IRE1 in protists sheds a new light on the evolution of the UPR. An alternative evolutionary history is conceivable.