Global mapping of protein subcellular location in apicomplexans: the parasite as we’ve never seen it before

Apicomplexans are human and animal protozoan pathogens responsible for diseases including malaria, cryptosporidiosis and toxoplasmosis. As obligate intracellular parasites they are highly organised cells with numerous novel and specialised sub-compartments that form the basis of their invasion biology, host defence evasion, and novel metabolic traits. However, our understanding of these cells is highly constrained by our limited knowledge of the locations and functions of most of the cell’s proteome. Even in the best-studied apicomplexans (Plasmodium spp. and Toxoplasma gondii) only a small fraction of proteins’ locations have been experimentally determined, with most assignments based on predictions from orthologues in distant relatives. Moreover, many parasite proteins are annotated as ‘hypotheticals’, for example 4113 of 8121 Toxoplasma proteins, and many are unique to parasites stymying even predictions of location or function by comparative biology. To address this deficit in our basic understanding of the compositional organisation of the apicomplexan cell, we have used a spatial proteomics method called hyper LOPIT to simultaneously capture the steady-state subcellular association of thousands of proteins in the apicomplexan Toxoplasma. These protein atlases reveal: extensive protein association networks throughout the cell providing testable hypotheses of their function; conservation and novelty of compartment proteomes between apicomplexans; differential selective pressures across the different cell compartments; and clear instances of protein relocation from one organelle to a different one during apicomplexan speciation. This new, global view of the organisation of the apicomplexan cell proteome provides a much more complete framework for understanding the mechanisms of function and biology of these cells.