Familial encephalopathy with neuroserpin inclusion bodies or FENIB is an autosomal dominant dementia that is characterized by intraneuronal inclusions of mutant neuroserpin. We report here the expression, purification and characterization of wildtype neuroserpin and neuroserpin containing the Ser49Pro mutation that causes FENIB. Wildtype neuroserpin formed SDS-stable complexes with tPA with an association rate constant and Ki of 1.2 x 104 M-1s-1 and 5.8 nM respectively. In contrast, Ser49Pro neuroserpin formed unstable complexes with an association rate constant and Ki of 0.3 x 104 M-1s-1 and 533.3 nM respectively. Assessment by circular dichroism showed that Ser49Pro neuroserpin had a lower melting temperature than wildtype protein (49.9 and 56.6 C respectively) and more readily formed loop-sheet polymers under physiological conditions. Neither the wildtype nor Ser49Pro neuroserpin accepted the P7-P2 α1-antitrypsin or P14-P3 antithrombin reactive loop peptides that have been shown to block polymer formation in other members of the serpin superfamily. Taken together these data demonstrate that Ser49Pro neuroserpin is a poor proteinase inhibitor and readily forms loop-sheet polymers. These findings provide strong support for the role of neuroserpin polymerization in the formation of the intraneuronal inclusions that are characteristic of FENIB. by guest, on January 28, 2013 www. jbc. org