Microtubule inner proteins in apicomplexan parasites

Microtubule inner proteins (MIPs) are integral components within the microtubule lumen of various organisms, contributing to microtubule structural integrity and functionality. Apicomplexan parasites, including Plasmodium spp. and Toxoplasma gondii, exhibit a range of specialized tubulin structures, such as axonemal microtubules, subpellicular microtubules (SPMTs), and conoid fibers, playing critical roles in cellular morphology and motility. Yet, in contrast with model organisms, only a few MIPs have been characterized in apicomplexans so far. Recent advances in cryo-electron tomography and structural proteomics have facilitated the study of MIPs, shedding light on unique adaptations that distinguish apicomplexan microtubules from those in model eukaryotes. Key findings include the identification of an interrupted luminal helix in SPMTs, which is critical for stabilizing microtubules under stress. The relatively small repertoire of axonemal MIPs contrasts markedly with the numerous MIPs observed in other systems, possibly reflecting adaptations for rapid microtubule assembly without intraflagellar transport. Furthermore, emerging evidence points to multiple MIPs within the conoid and SPMTs, suggesting further roles for MIPs in these parasites. This review highlights the currently known contributions of MIPs to the survival and proliferation of these parasites, while emphasizing the need for continued research to fully characterize their diverse roles and molecular mechanisms.