Discussion
Malaria-causing Plasmodium parasites remain a major global health threat, relying on rapid host-cell invasion to complete their complex life cycle. This process is driven by the glideosome, a conserved actomyosin motor complex anchored within the inner membrane complex. While the core motor components of the glideosome have been extensively characterised, how this machinery is structurally anchored within the IMC and mechanically coupled to the parasite cytoskeleton has remained unknown. Members of the GAPM family have long been proposed to occupy this interface; however, their organisation, functional relationships, and localisation across the parasite life cycle have remained poorly understood. Here, we show that GAPM proteins are expressed throughout the Plasmodium life cycle with GAPM2 localisation is tightly coordinated with nuclear division, parasite segmentation, and apical specification, suggesting a fundamental role in IMC biogenesis and organisation. To define the molecular architecture of GAPMs, we reconstituted the complex and solved its structure by cryo-electron microscopy. We find that GAPM1, GAPM2, and GAPM3 assemble into a stable, obligate 1:1:1 heterotrimer. Our cryo-EM structure reveals that each subunit contributes six transmembrane helices, generating a trimeric core with asymmetric features that create a multi-surfaced platform that may accommodate diverse binding partners. Consistent with this model, GAPM2 pull-downs from Plasmodium parasites reveal that the GAPM complex associates with actin–myosin motor components during asexual blood stages, but not during early sexual development. These data suggest that while GAPMs form a stable core complex, their functional interactions are developmentally regulated. Together, our findings establish GAPMs as a heterotrimeric membrane scaffold within the IMC and support a model in which this complex provides an anchoring platform for the Plasmodium glideosome.
