Submembranous septins as relatively stable components of actin-based membrane skeleton

The cell cortex is organized by the dynamic interplay between the plasma membrane, membrane proteins, and the cytoskeleton. Despite the cortical localization of septin heteropolymers in vivo and their direct interaction with phospholipid membranes in vitro, their behavior and roles remain elusive. This study characterizes the major cortical septin assembly found in mammalian tissue culture cells by fluorescence recovery after photobleaching analysis. GFP-tagged septin subunits, which colocalized with cortical actin, exhibited slower turnover than some other cortical proteins that were analyzed (e.g., actin, syntaxin-1A and a glutamate aspartate transporter [GLAST]). Perturbation of actin turnover by cytochalasin D or jasplakinolide retarded the cortical septin turnover, while septin depletion by RNAi did not recognizably affect cortical actin turnover. These phenomena are compatibly interpreted by septins' selective association with a subset of actin-based membrane skeleton, as revealed by rapid-freeze deep-etch immuno-replica electron microscopy. We applied the assay system to test septins' presumptive scaffold function on their physiological binding partners. Septin filament destabilization by RNAi-mediated subunit depletion facilitated the turnover of GLAST, depending on the carboxyl-terminal 29 residues, while a septin filament-stabilizing drug forchlorfenuron restrained more GLAST in the unexchangeable fraction. These data indicate that cortical septin heteropolymers are components of the actin-based membrane skeleton providing scaffolds for their interacting partners probably by impeding their lateral diffusion. We predict that diverse submembranous septin clusters found in vivo may serve as scaffolds or reserve pools for specific membrane-bound proteins.