A mutant αII-spectrin designed to resist calpain and caspase cleavage questions the functional importance of this process in vivo

alpha- and beta-spectrins are components of molecular scaffolds located under the lipid bilayer and named membrane skeletons. Disruption of these scaffolds through mutations in spectrins demonstrated that they are involved in the membrane localization or the maintenance of proteins associated with them. The ubiquitous alpha II-spectrin chain bears in its central region a unique domain that is sensitive to several proteases such as calpains or caspases. The conservation of this region in vertebrates suggests that the proteolysis of alpha II-spectrin by these enzymes could be involved in important functions. To assess the role of alpha II-spectrin cleavage in vivo, we generated a murine model in which the exons encoding the region defining this cleavage sensitivity were disrupted by gene targeting. Surprisingly, homozygous mice expressing this mutant alpha II-spectrin appeared healthy, bred normally, and had no histological anomaly. Remarkably, the mutant alpha II-spectrin assembles correctly into the membrane skeleton, thus challenging the notion that this region is required for the stable biogenesis of the membrane skeleton in nonerythroid cells. Our finding also argues against a critical role of this particular alpha II-spectrin cleavage in either major cellular functions or in normal development.