Actin‐like protein 1 (ALP1) is a component of dynamic, high molecular weight complexes in Toxoplasma gondii

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Abstract
Apicomplexan parasites, such as Toxoplasma gondii, rely on actin‐based motility for cell invasion, yet conventional actin does not appear to be required for cell division in these parasites. Apicomplexans also contain a variety of actin‐related proteins (Arps); however, most of these not directly orthologous to Arps in well‐studied systems. We recently identified an apicomplexan‐specific member of this family called Actin‐Like Protein 1, (ALP1), which plays a role in the assembly of vesicular components recruited to the inner membrane complex (IMC) of daughter cells during cell division. In addition to its enrichment at daughter cell membranes, ALP1 is localized throughout the cytoplasm both diffusely distributed and concentrated in clusters that are detected by fluorescence microscopy, suggesting it forms complexes. Using quantitative optical imaging methods, including fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP), we demonstrated that ALP1 is a component of a large complex, and that it readily exchanges between diffusible and complex‐bound forms. Sedimentation and density gradient analyses revealed that ALP1 is found in a freely soluble state as well as high molecular weight complexes. During cell division, ALP1 was dynamically associated with the IMC, suggesting it rapidly cycles between freely diffusible and complex forms during daughter cell assembly.