Structural requirements for VAP-B oligomerization and their implication in ALS-associated VAP-B(P56S) neurotoxicity.

J Biol Chem 285(18):13839-13849 (2010) PMID 20207736 PMCID PMC2859547

The integral ER-membrane protein VAP-B interacts with various lipid-transfer/binding proteins containing a FFAT-motif through its N-terminal MSP domain. A genetic mutation within its MSP domain, P56S, was identified in familial forms of motor neuron diseases (MNDs). This mutation induces the formation of insoluble VAP-B(P56S) protein aggregates by an unknown mechanism. In this study, we defined the structural requirements for VAP-B oligomerization and demonstrated their contribution for VAP-B(P56S) aggregation and neurotoxicity. We show that the oligomerization of VAP-B is mainly mediated by its coiled-coil domain, and that the GXXXG dimerization motif within the transmembrane domain (TMD) mediates TMDs self-association, but is insufficient to drive VAP-B oligomerization. We further show that the oligomerization of the wild-type VAP-B is independent of its MSP domain. However, we found that the P56S mutation induces conformational changes within the MSP domain and facilitates its propensity to aggregate by exposing hydrophobic patches to the solvent. These conformational changes have no direct effect on FFAT-binding. Rather, they enhance VAP-B(P56S) oligomerization driven by the combined contributions of the coiled-coil and the transmembrane domains, thereby preventing accessibility to FFAT-binding site, facilitating the production of VAP-B(P56S) insoluble aggregates and consequently its neurotoxicity. These results shed light on the mechanism by which VAP-B(P56S) aggregates are formed and induce familial MNDs.