Abstract
Most apicomplexan parasites harbor a relict chloroplast, the apicoplast, that is critical for their survival. Whereas the apicoplast maintains a small genome, the bulk of its proteins are nuclear encoded and imported into the organelle. Several models have been proposed to explain how proteins might cross the four membranes that surround the apicoplast; however, experimental data discriminating these models are largely missing. Here we present genetic evidence that apicoplast protein import depends on elements derived from the ER-associated protein degradation (ERAD) system of the endosymbiont. We identified two sets of ERAD components in Toxoplasma gondii, one associated with the ER and cytoplasm and one localized to the membranes of the apicoplast. We engineered a conditional null mutant in apicoplast Der1, the putative pore of the apicoplast ERAD complex, and found that loss of Der1(Ap) results in loss of apicoplast protein import and subsequent death of the parasite.
Highlights
Endosymbiosis is well established as a mechanism that has played a crucial role in the evolution of eukaryotic cells
We found that the two T. gondii early in organelle acquisition, and a simple solution would have proteins were of divergent phylogenetic origin (Fig. 6)
Our previous cytoplasmic protein forms a well-supported clade with studies support this model and have shown that transport over homologs from chromalveolates
Summary
We demonstrated that a homolog of Tic, a component of the translocon of the inner chloroplast membrane (Tic) complex in plants, is likely required for protein import across the innermost membrane of the apicoplast [12]. Analysis of the genome of the remnant nucleus of the algal endosymbiont in cryptomonads showed the presence of an ER-associated protein degradation (ERAD) system and offered a candidate for a translocon across the third and potentially the second membrane [14]. Der homologs have been shown to localize to the plastids of a range of secondary plastid-containing organisms [14, 19, 20], but evidence linking these proteins to a functional role in apicoplast import is lacking. We provide direct genetic and biochemical evidence for an essential function of the ERAD membrane component Der1Ap in apicoplast protein import
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