Charakterisierung eines minimierten eukaryoten Cytoplasmas - das periplastidäre Kompartiment der Diatomee Phaeodactylum tricornutum

Diatoms maintain two evolutionary different cytoplasms: (i) the cytosol of the host cell and (ii) the periplastidal compartment (PPC), which resembles the reduced cytoplasm of a red algal endosymbiont (complex plastid). The PPC is present in a multitude of ecological highly relevant microalgae and pathogenic protists (Apicomplexa) that contain complex plastids surrounded by four membranes. Knowledge about the protein composition and function of the PPC as yet was only sparse. Therefore in this study the protein composition of the PPC of the diatom Phaeodactylum tricornutum was analyzed using a combined in silico/in vivo approach. PPC candidates were identified in the genome by means of specific N-terminal targeting sequences followed by expression of eGFP fusion proteins to investigate their subcellular localizations. This screen resulted in the identification of a total of 29 new PPC proteins, including further proteins for already known functions like protein transport (SELMA), protein folding and metabolism, as well as factors indicating new functions in the PPC. These cover protein degradation and processing, plastid division, lipid transfer and a potential structural organization. Equally important were processes genomically not detectable in the PPC, which was true for typical eukaryotic vesicular and cytoskeleton components as well as proteins for classical signal transduction. The results of this work indicate that the PPC is a naturally minimized eukaryotic cytoplasm with reduced metabolic and cellular capacities showing less similarity to an autonomous eukaryotic cytoplasm. Furthermore the results suggest that the PPC is mainly involved in maintenance of the former primary plastid and its own integrity. A further topic of this study dealt with the identification and localization of metabolite translocators that might generate a link between metabolic symbiont and host pathways. A combination of in silico analyses and in vivo localization studies succeeded in detection of several putative triosephosphate/phosphate translocators in the membranes of the complex plastid of P. tricornutum. The results indicate that these proteins could facilitate a metabolic communication of host and symbiont, possibly by transport of triosephosphates.