Spectrin condensates provide a nidus for assembling the axonal membrane-associated periodic skeleton.

In axons, α/β-spectrins, adducin, and actin filaments assemble into a lattice underneath the plasma membrane, but the mechanistic events leading to this membrane-associated periodic skeleton (MPS) are unclear. Visualizing MPS-components in developing axons, we found distal focal patches containing spectrins and adducin (but sparse actin filaments) with biophysical properties reminiscent of biomolecular condensation. Overexpressing spectrin repeats - constituents of α/β-spectrins - in heterologous cells triggered condensate formation, and preventing the association of βII-spectrin with actin filaments or membranes also facilitated condensation. Introducing a stretch of spectrin-repeats in neurons before MPS establishment triggered ectopic condensate-like structures in the soma and disrupted the axonal lattice, advocating a functional role for biomolecular condensation. We propose a condensation-assembly model where spectrin-repeats trigger focal phase separated condensates, providing a nidus for MPS assembly that recruits actin filaments to ultimately generate the stable lattice. Our overall model is supported by recent studies showing phase-separation via coiled-coil domains and recruitment/polymerization of actin by other condensate-forming proteins.