Proteintransport zum Apicoplasten von Plasmodium falciparum - Hinweise für einen Golgi-abhängigen Transportweg

Like most other apicomplexan parasites Plasmodium falciparum harbours a complex, four-membrane-bound plastid termed the apicoplast that has been acquired through a secondary endosymbiosis event. Although no longer photosynthetically active, the apicoplast is the site of essential biochemical pathways and therefore considered a potential drug target for the next generation of anti-malarials. However, due to its endosymbiotic origin, most genes encoding apicoplast proteins have been transferred to the parasite nucleus during the course of evolution and therefore are in need of signals and transport mechanisms that facilitate post-translational re-import of their products to the complex plastid. Whilst signals mediating co-translational insertion into the ER and further transport to plastid, as well as components facilitating the protein translocation across the three inner membranes of the apicoplast have been studied in detail during the last years, it remains largely unknown how proteins are initially trafficked from the ER to the outermost apicoplast membrane. Most models favour a direct ER to apicoplast trafficking route, in which proteins destined for the apicoplast are recognised in the ER and diverted to a specialised vesicular system that carries them to the outer apicoplast membrane thereby bypassing the Golgi-compartment. This work provides substantial evidence that soluble proteins en route to the apicoplast indeed travel through the Golgi-compartment before reaching their final destination. By using ER retrieval sequences as Golgi-specific recognition signals we find that upon addition to an apicoplast-targeted reporterprotein, transport of this protein to the apicoplast is significantly impaired. Furthermore, trafficking of a soluble apicoplast-targeted reporterprotein can be substantially inhibited by brefeldin A, a fungal metabolite known to interfere with vesicle transport at the Golgi/ER interface, in a P. falciparum wildtype strain, whereas in a BFA-resistant P. falciparum strain transport is not affected. We propose that, upon co-translational insertion into the ER, soluble apicoplast proteins initially follow the default secretory pathway. In the Golgi they are then recognised, most likely by a putative transitpeptide receptor, and diverted to a vesicular system that directs them to the outer apicoplast membrane.