The apicomplexan parasites, Plasmodium spp., and Toxoplasma gondii, have a non-photosynthetic plastid that originated through secondary endosymbiosis and remains an essential organelle for parasite survival. Although the apicoplast possesses its own small genome, the vast majority of its proteins are encoded in the parasite nucleus and must be accurately transported to the organelle after synthesis in the cytoplasm. Targeting of these proteins is mediated by a characteristic N-terminal bipartite targeting sequence consisting of a signal peptide followed by a transit peptide. The signal peptide directs newly synthesized proteins into the endoplasmic reticulum (ER), after which they are sorted and transported to the apicoplast.

The lab has made significant contributions toward understanding the mechanisms that govern apicoplast protein trafficking. Through a combination of cell biological, molecular, and biochemical approaches, we demonstrated that apicoplast proteins do not follow a single trafficking route. Instead, at least two distinct pathways exist. Proteins that are exclusively localised to the apicoplast primarily utilise a Golgi-independent pathway, allowing direct transport from the ER to the organelle. In contrast, proteins that are dually localized to both the apicoplast and mitochondria are trafficked through a Golgi-dependent route. Our work further revealed that the determinants of pathway selection extend beyond the N-terminal bipartite signal sequence. The coding sequence of the protein itself also contributes to trafficking decisions, indicating that multiple layers of information within the protein cooperate to ensure accurate targeting.

Building on these findings, our current research focuses on several fundamental unanswered questions in apicoplast biology. One major objective is to identify the molecular machinery that recognizes the transit peptide at the ER and commits proteins to the apicoplast trafficking pathway in T. gondii. Despite the central role of transit peptides in organelle targeting, the receptors and sorting factors responsible for this recognition remain unknown. We have used a proximity-based protein labelling approach called BioID to identify proteins that interact with the transit peptide in the ER or Golgi, and are currently characterizing them.

While components involved in translocation across the inner membranes of the apicoplast have been characterized, the mechanisms that mediate the initial entry of proteins into this four-membrane-bound organelle remain poorly understood. The apicoplast-localized thioredoxin ATrx1 follows a Golgi-independent trafficking route and has been suggested to play a role in the import of proteins into the apicoplast. Building on this observation, we have identified candidate proteins that may participate in this import pathway.

Funded by: Board for Research in Nuclear Sciences (2013/37B/18/BRNS) and Department of Biotechnology (BT/PR13546/BRB/10/1423/2015)

Apicoplast trafficking team