Strumpellin and Spartin, Hereditary Spastic Paraplegia Proteins, are Binding Partners

Hereditary spastic paraplegia (HSP) is one of the most heterogeneous neurodegenerative diseases with more than 50 identified genes causing a relatively stereotypical phenotypic presentation. Recent studies of HSP pathogenesis have suggested the existence of shared biochemical pathways that are crucial for axonal maintenance and degeneration. We explored possible interactions of several proteins associated with this condition. Here we report interactions of endogenous and overexpressed strumpellin with another HSP-associated protein, spartin. This biochemical interaction does not appear to be a part of the Wiskott-Aldrich syndrome protein and Scar homologue (WASH) complex because spartin is not co-immunoprecipitated with WASH1 protein. The spartin-strumpellin association does not require the presence of the microtubule interacting and trafficking domain of spartin. Over-expression of mutant forms of strumpellin with the introduced HSP-causing mutations does not alter the colocalization of these two proteins. Knockdown of strumpellin in cultured cortical rat neurons interferes with development of neuronal branching and results in reduced expression of endogenous spartin. Proteosomal inhibition stabilized the levels of spartin and WASH1 proteins, supporting increased spartin degradation in the absence of strumpellin.