Nuclear localization of cystatin B, the cathepsin inhibitor implicated in myoclonus epilepsy (EPM1)

Cystatin B is an anti-protease implicated in myoclonus epilepsy, a degenera tive disease of the central nervous system. In vitro, cystatin B interacts with and inhibits proteases of the cathepsin family. Confocal microscopy an alysis of the subcellular localization of cystatin B and cathepsin B shows that, in vivo the two proteins are concentrated in different cell compartme nts. In fact, cystatin B is found mainly in the nucleus of proliferating ce lls and both in the nucleus and in the cytoplasm of differentiated cells, w hile cathepsin B, in either case, is essentially cytoplasmic. However, colo calization of cystatin and cathepsin B is observed in the isolated cell mat rix and in the nuclear scaffold of differentiated neuroblastoma cells but n ot of proliferating cells. This suggests that at least a fraction of cystat in B is bound to the protease in differentiated cells. The electron microsc opy analysis of the cell matrix confirms the observation made with confocal microscopy. The cellular activity of cathepsin B was analyzed with a fluor ogenic cytochemical assay. A fluorescent signal is observed in the cytoplas m of proliferating cells but is undetectable in the cytoplasm of differenti ated cells, suggesting that cathepsin B is active mainly during the cell cy cle. This result is consistent with the separate compartmentalization of cy statin B and cathepsin B that we have observed in growing cells. (C) 2001 A cademic Press.