Table of Contents:
Diatoms harbor a complex plastid of red algal origin, which probably has emerged from a polyphy-letic event respectively an endosymbiosis of higher order. A Rhodophyta was successively estab-lished as endosymbiont to a semi-autonomous complex plastid surrounded by four enveloping membranes. In contrast to Crypto- and Chlorarachniophyta, essential symbiont-specific genes have been fully integrated into the host genome of Heterokonts, Haptophyta, Apicomplexa and Peridinin-pigmented Dinoflagellata, thereby eliminating the former symbiontic nucleus.
The encoding of plastid proteins on the host nucleus required the establishment of new protein translocation mechanisms including whose specific targeting sequences. It is postulated that plas-tid preproteins at the outermost plastid membrane of complex plastids of rhodophyte origin are translocated into the plastid lumen (cER) via Sec61. The transport along the second plastid mem-brane (PPM) is rather unclear. It has been shown that the transport of plastidary preproteins at the PPM can take place using the so-called symbiont-specific ERAD-like machinery (SELMA). For this system, which probably derived from the ER-associated degradation, ERAD homologues have been identified and specific interactions have been shown in the past few years. Thus, SELMA very likely represents a system recycled during evolution and adapted to new tasks.
A postulated, not yet identified import receptor could recognize preproteins as a soluble cER pro-tein or as a PPM-resident protein and therefore recruit this plastid preproteins at the second plas-tid membrane for further translocation. In this work, candidates for the postulated import recep-tor were sought by two directed in silico approaches as well as due to the native protein separa-tion of the SELMA-complex and its subsequent massspectrometrical analysis. In total, 32 of these proteins with eGFP at their C-terminus, were localized in the diatom Phaeodactylum tricornutum. Eight of these fusionproteins are possibly localized in the secretory pathway, three other candi-dates localize very likely in mitochondria. Furthermore, two putative PPM-resident, nine possibly cER localized and eight potentially PPC fusionproteins were identified in this work. For two of the PPC fusionproteins their PPC-localization could be verified. A putative ER lumen receptor shows plastid localization (cER) and therefore could be a putative import receptor. Additionally, specific interactions for one PPC-resident TPR-fusionprotein with two mitochondrial carrier proteins of the complex plastid and the PPM-protein sDer1-1 were shown in vivo.