ORP-3 Rescues ER Membrane Expansions Caused by the VAPB-P56S Mutation in Familial ALS

Abstract

A mutation in ER membrane protein VAPB is responsible for causing a familial form of ALS (ALS8). The VAPB-P56S mutation causes protein aggregation and a nuclear envelope defect, where retrograde transport is disrupted. Over-expression of a FFAT peptide from OSBP1 reduces the size of VAPB-P56S aggregates and restores retrograde transport. A screen was performed on FFAT-motif containing ORPs to determine if any could rescue the mutant phenotype. ORP3 successfully reduced aggregate size and restored transport to the nuclear envelope. ER membrane protein Sac1, a PI4P phosphatase cycles between the ER and Golgi and becomes trapped in expanded ERGIC compartments with VAPB-P56S. Loss of Sac1 in the ER leads to an increase in intracellular PI4P. ORP3 may increase Sac1 phosphatase activity by acting as a lipid sensor. We propose that VAPB, Sac1 and ORP3 are interacting partners that together modulate levels of PI4P. Disruptions in the gradient of PI4P may result in the vesicle trafficking defects observed in VAPB-P56S cells.