Neuroserpin is a member of the
serpin superfamily. Point mutations in the
neuroserpin gene underlie the autosomal dominant
dementia, familial encephalopathy with
neuroserpin inclusion bodies. This is characterized by the retention of ordered
polymers of
neuroserpin within the endoplasmic reticulum of neurons. pH has been shown to affect the propensity of several
serpins to form
polymers. In particular, low pH favors the formation of
polymers of both alpha(1)-antitrypsin and
antithrombin. We report here opposite effects in
neuroserpin, with a striking resistance to
polymer formation at acidic pH. Mutation of specific
histidine residues showed that this effect is not attributable to the shutter domain
histidine as would be predicted by analogy with other
serpins. Indeed, mutation of the shutter domain His338 decreased
neuroserpin stability but had no effect on the pH dependence of polymerization when compared with the wild-type
protein. In contrast, mutation of His119 or His138 reduced the polymerization of
neuroserpin at both acidic and neutral pH. These residues are at the lower pole of
neuroserpin and provide a novel mechanism to control the opening of beta-sheet A and hence polymerization. This mechanism is likely to have evolved to protect
neuroserpin from the acidic environment of the secretory granules.