Evolution and structural plasticity of microvilli

Actin-based protrusions shape cellular morphology and mediate environmental sensing, yet the evolutionary timeline and structural principles governing their organization are not fully understood. Here, we investigate the unicellular frontier of metazoan multicellularity by resolving the native architecture of microvilli in the choanoflagellate Salpingoeca rosetta using in situ cryo-electron tomography and single-particle cryo-electron microscopy. We discover that these premetazoan microvilli are sustained by a highly ordered, hexagonal actin array cross-bridged by SrFascin and decorated by SrLasp,an ancestral member of the Nebulin-family protein. Structural analysis reveals that SrLasp engages F-actin through a binding mechanism conserved in vertebrate skeletal muscle Nebulin, suggesting that this mode of actin regulation predates the emergenc of animals. Beyond this profound evolutionary conservation, the microvillar cytoskeleton exhibits remarkable structural plasticity to accommodate geometric constraints. Fascin cross-bridges adjacent filaments within the 7-filament core through discrete "skip-or-flip" shifts in crossbridging register, accommodating the intrinsic helical mismatch between neighboring actin filaments. Together, these findings delineate a primordial actin-organizing module for microvillar architecture and illuminate the evolutionary origins of Nebulin-family actin regulation before the emergence of animals.