Infantile-onset ascending hereditary spastic paralysis (IAHSP) is a rare, early-onset autosomal recessive motor neuron disease, caused by mutations in the ALS2 gene encoding for the Alsin protein. Alsin is 184kDa protein composed of several independent structured domains implicated in cell signalling, membrane transport events and cytoskeletal reorganization in mammalian neuronal cell. Most of the associated disease mutations of Alsin are localized in its C-terminal region, leading to a loss of function or cellular mislocalization of the entire protein. This region is characterized by a MORN motif followed by a VPS9 domain: in vitro and in vivo studies on VPS9 domain of Alsin, showed that this domain activates Rab5 GTPase owing to its guanine exchange factor (GEF) activity on this small GTPase family. Together with MORN, VPS9 mediates the homo-oligomerization and localization of Alsin in neuron axons. Recently a missense mutation (R1611W) of VPS9 domain has been identified to lead to the development of IAHSP. To date, no structural information is currently available neither on ALS2 gene product nor on any of its domains. We aim at providing a high-resolution structure and a biophysical characterization of the native and mutated VPS9 domain.
Biallelic mutations of the Alsin (ALS2) gene cause an autosomal recessive neurodegenerative disorder named Infantile onset ascending hereditary spastic paralysis (IAHSP), characterized by retrograde degeneration of the upper motor neurons of the pyramidal tracts(1). ALS2 gene product is a multi-domain protein implicated in cell signalling, membrane transport events and cytoskeletal reorganization in mammalian neuronal cells(3). Alsin a member of the Vps9 domain-containing family of proteins and in addition to its C-terminal Vps9 domain, contains a region similar to the b-propeller of the Ran GEF RCC1 (RCC1-like domain), tandem DH and PH domains indicative of a Rho family GEF, and several MORN (membrane occupation and recognition nexus) motifs. The Vps9 domain of Alsin specifically catalyses guanine nucleotide exchange on Rab5, whose activation is essential in the early stages of endocytic pathway. MORN function is still unclear, but studies suggest its role in driving ALS2 endosomes localization and homo-oligomerization(2).
Although computational studies on Alsin have been made to predict the structure of its domains, no experimental structures are currently available. Here we propose to lead a structural and biochemical characterization, of the VPS9 domain, in its native and mutated form that could unravel molecular mechanisms underlying physiological or pathological functions in the cell leading to several neurodegenerative disorders.
(1) Helal, M. et al. Neurol. Sci. (2018)
(2) Sato, K. et al. J. Biol. Chem. (2018)
(3) Topp, J. D. et al. J. Biol. Chem. (2004)