Multivalent interactions drive the Toxoplasma AC9:AC10:ERK7 complex to concentrate ERK7 in the apical cap

The Toxoplasma inner membrane complex (IMC) is a specialized organelle that is crucial for the parasite to establish an intracellular lifestyle and ultimately cause disease. The IMC is composed of both membrane and cytoskeletal components, further delineated into the apical cap, body, and basal subcompartments. The apical cap cytoskeleton was recently demonstrated to govern the stability of the apical complex, which controls parasite motility, invasion, and egress. While this role was determined by individually assessing the apical cap proteins AC9, AC10, and the MAP kinase ERK7, how the three proteins collaborate to stabilize the apical complex is unknown. In this study, we use a combination of deletion analyses and yeast-2-hybrid experiments to establish that these proteins form an essential complex in the apical cap. We show that AC10 is a foundational component of the AC10:AC9:ERK7 complex and demonstrate that the interactions among them are critical to maintain the apical complex. Importantly, we identify multiple independent regions of pairwise interaction between each of the three proteins, suggesting that the AC9:AC10:ERK7 complex is organized by multivalent interactions. Together, these data support a model in which multiple interacting domains enable the oligomerization of the AC9:AC10:ERK7 complex and its assembly into the cytoskeletal IMC, which serves as a structural scaffold that concentrates ERK7 kinase activity in the apical cap. ### Competing Interest Statement The authors have declared no competing interest.