Repeated secondary loss of adaptin complex genes in the Apicomplexa

The Apicomplexa include parasites of devastating medical and economic consequence. While obviously essential for their parasitic mechanism, the molecular machinery underpinning membrane-trafficking in many apicomplexans is poorly understood. One potentially key set of players, the adaptins, selects cargo for incorporation into trafficking vesicles. Four distinct adaptin (AP) complexes exist in eukaryotes; AP1 and AP3 are involved in transport between the trans-Golgi Network (TGN) and endosomes, AP4 in TGN to cell surface transport, and AP2 in endocytosis from the cell surface. Of particular interest is the involvement of AP1 in Toxoplasma rhoptry biogenesis. The recent completion of several apicomplexan genomes should jump-start molecular parasitological studies and provide systems-level insight into the apicomplexan adaptin machinery. However, many of the encoded adaptin proteins are annotated conservatively and not to the necessary complex or subunit level. Prompted by previous evidence suggesting the lack of AP3 in Plasmodium falciparum, we undertook homology-searching and phylogenetic analysis to produce a rigorously annotated set of adaptin subunits encoded in diverse apicomplexan genomes. We found multiple losses of adaptins across the phylum; in particular Theileria, Babesia, and Cryptosporidium, but surprisingly not Plasmodium, appear to have lost the entirety of the AP3 complex. The losses correlate with a degenerate Golgi body structure and are reminiscent of recently reported secondary losses of additional endocytic components (i.e. the ESCRTs) in several Apicomplexa. These data may indicate a relaxation of the selective pressure on the apicomplexan endocytic system and, regardless, should greatly facilitate future molecular cell biological investigation of the role of adaptins in these important parasites.