VAP-B (Vesicle-Associated Membrane Protein-Associated Protein B) is a ubiquitously expressed, ER-resident tail-anchored adaptor protein implicated in interorganellar lipid exchange, generation of membrane contact sites and membrane traffic. Its mutant form, P56S-VAPB, has been linked to a dominantly inherited form of amyotrophic lateral sclerosis (ALS8). P56S-VAPB forms intracellular inclusions, whose role in ALS pathogenesis has not yet been elucidated. We show that newly synthesized mutant VAPB is normally targeted to the ER where it rapidly aggregates, generating a restructured ER domain consisting of ribbons of stacked cisternae apparently held together by the mutated cytosolic domain of VAPB. The P56S-VAPB structures are continuous with the surrounding normally organized ER, which is devoid of the mutant protein, and are formed also when the mutant protein is expressed at physiological levels. At variance with most pathological inclusion bodies, P56S-VAPB inclusions undergo turnover, and the rate of degradation of the aggregated polyubiquitinated mutant is faster than that of the wild type protein. Degradation of the mutant protein involves the p97 ATPase and the proteasome with no apparent participation of macro-autophagy, at least in basal conditions. Enhanced degradation of P56SVAPB can only be observed when autophagy is stimulated by starvation, although autophagocytosis does not appear to be slowed in cells expressing mutant VAPB. Involvement of the proteasome in P56S-VAPB clearance does not impair the proteasome's ability to clear a classical ERAD substrate. Our results reveal surprisingly efficient extraction from the ER and proteasomal degradation of a severely aggregated mutant protein. Moreover, they suggest that the slow onset of P56S-linked familial ALS is not a consequence of the progressive accumulation of the mutant protein over time, and that the dominant inheritance of the mutant allele may rather be related to haploinsufficiency.
Structure, biogenesis, and clearance of ER-derived inclusions generated by the ALS-linked mutant of VAPB
Francesca Navone;Nica Borgese
2013
Abstract
VAP-B (Vesicle-Associated Membrane Protein-Associated Protein B) is a ubiquitously expressed, ER-resident tail-anchored adaptor protein implicated in interorganellar lipid exchange, generation of membrane contact sites and membrane traffic. Its mutant form, P56S-VAPB, has been linked to a dominantly inherited form of amyotrophic lateral sclerosis (ALS8). P56S-VAPB forms intracellular inclusions, whose role in ALS pathogenesis has not yet been elucidated. We show that newly synthesized mutant VAPB is normally targeted to the ER where it rapidly aggregates, generating a restructured ER domain consisting of ribbons of stacked cisternae apparently held together by the mutated cytosolic domain of VAPB. The P56S-VAPB structures are continuous with the surrounding normally organized ER, which is devoid of the mutant protein, and are formed also when the mutant protein is expressed at physiological levels. At variance with most pathological inclusion bodies, P56S-VAPB inclusions undergo turnover, and the rate of degradation of the aggregated polyubiquitinated mutant is faster than that of the wild type protein. Degradation of the mutant protein involves the p97 ATPase and the proteasome with no apparent participation of macro-autophagy, at least in basal conditions. Enhanced degradation of P56SVAPB can only be observed when autophagy is stimulated by starvation, although autophagocytosis does not appear to be slowed in cells expressing mutant VAPB. Involvement of the proteasome in P56S-VAPB clearance does not impair the proteasome's ability to clear a classical ERAD substrate. Our results reveal surprisingly efficient extraction from the ER and proteasomal degradation of a severely aggregated mutant protein. Moreover, they suggest that the slow onset of P56S-linked familial ALS is not a consequence of the progressive accumulation of the mutant protein over time, and that the dominant inheritance of the mutant allele may rather be related to haploinsufficiency.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
