Die Evolution des Proteinimports in die komplexen Plastiden der Chromalveolaten

Chromalveolates are a diverse protist group. The phototrophic chromalveolates as well as the apicomplexan parasites possess an organelle – termed the complex plastid – tracing back to an engulfed Rhodophyte. The work presented here increases the understanding of the evolution of protein import in red complex plastids. (1) The haptophyte Emiliania huxleyi expresses a SELMA system. Thus, SELMA exists in all chromalveolates with plastids surrounded by four membranes. Moreover, in vivo localization studies of haptophytic sequences in the diatom P. tricornutum are feasible. Phylogenetic analyses of h/sCdc48 and h/sUba1 indicate a red-algal origin of SELMA. ERAD sequences do not support monophyly and therefore contradict the chromalveolate hypothesis. (2) The identification of the deubiquitinase ptDUP indicates a mechanistic conservation of SELMA mediated transport. PtDUP is a PPC resident enzyme proven to be functional in vitro. Presumably, ptDUP deconjugates preproteins imported into the PPC ensuring their proper maturation or further transport steps into the complex plastid. (3) Nucleus encoded plastid proteins rely on targeting signals for import. It was shown that this is dependent on positive charge in diatoms. The crucial amino acids may not only be resident within the physical transit peptide, but also within the mature protein domain. This result has imüplications for endosymbiotic gene transfer, since the acquisition of proper targeting signals could be more efficient in these cases, because the demands for targeting signals within the presequence are lowered. Transit peptides, that are not self-sufficient, could have been recruited for imported to those genes encoding intrinsic targeting motifs. The self-sufficiency of the transit peptide domain could have been evolved in later steps by subsequent mutations.