Molecular consequences of SCA5 mutations in the spectrin-repeat domains of beta-III-spectrin

Abstract

Spinocerebellar ataxia type 5 (SCA5) mutations in the protein {beta}-III-spectrin cluster to the N-terminal actin-binding domain (ABD) and the central spectrin-repeat domains (SRDs). We previously reported that a common molecular consequence of ABD-localized SCA5 mutations is increased actin binding. However, little is known about the molecular consequences of the SRD-localized mutations. It is known that the SRDs of {beta}-spectrin proteins interact with -spectrin to form an /{beta}-spectrin dimer. In addition, it is known that SRDs neighbouring the {beta}-spectrin ABD enhance actin binding. Here, we tested the impact of the SRD-localized R480W and the E532_M544del mutations on the binding of {beta}-III-spectrin to -II-spectrin and actin. Using multiple experimental approaches, we show that both the R480W and E532_M544del mutants can bind -II-spectrin. However, E532_M544del causes partial uncoupling of complementary SRDs in the /{beta}-spectrin dimer. Further, the R480W mutant forms large intracellular inclusions when co-expressed with -II-spectrin in cells, supporting that R480W mutation grossly disrupts the -II/{beta}-III-spectrin physical complex. Moreover, actin-binding assays show that E532_M544del, but not R480W, increases {beta}-III-spectrin actin binding. Altogether, these data support that SRD-localized mutations alter key interactions of {beta}-III-spectrin with -II-spectrin and actin.