BetaIV spectrins are essential for membrane stability and the molecular organization of nodes of Ranvier.

High densities of sodium channels at nodes of Ranvier permit action potential conduction and depend on betaIV spectrins, a family of scaffolding proteins linked to the cortical actin cytoskeleton. To investigate the molecular organization of nodes, we analyzed qv(3J)"quivering" mice, whose betaIV spectrins have a truncated proline-rich "specific" domain (SD) and lack the pleckstrin homology (PH) domain. Central nodes of qv(3J) mice, which lack betaIV spectrins, are significantly broader and have prominent vesicle-filled nodal membrane protrusions, whereas axon shape and neurofilament density are dramatically altered. PNS qv(3J) nodes, some with detectable betaIV spectrins, are less affected. In contrast, a larger truncation of betaIV spectrins in qv(4J) mice, deleting the SD, PH, and ankyrinG binding domains, causes betaIV spectrins to be undetectable and causes dramatic changes, even in peripheral nodes. These results show that quivering mutations disrupt betaIV spectrin retention and stability at nodes and that distinct protein domains regulate nodal structural integrity and molecular organization.