DISTINCT NUCLEAR-LOCALIZATION AND ACTIVITY OF TISSUE TRANSGLUTAMINASE

Tissue transglutaminase is a calcium-dependent transamidating enzyme t hat has been postulated to play a role in the pathology of expanded CA G repeat disorders with polyglutamine expansions expressed within the affected proteins. Because intranuclear inclusions have recently been shown to be a common feature of many of these codon reiteration diseas es, the nuclear localization and activity of tissue transglutaminase w as examined. Subcellular fractionation of human neuroblastoma SH-SY5Y cells demonstrated that 93% of tissue transglutaminase is localized to the cytosol, Of the 7% found in the nucleus, 6% copurified with the c hromatin-associated proteins, and the remaining 1% was in the nuclear matrix fraction. In situ transglutaminase activity was measured in the cytosolic and nuclear compartments of control cells, as well as cells treated with the calcium-mobilizing agent maitotoxin to increase endo genous tissue transglutaminase activity. These studies revealed that t issue transglutaminase was activated in the nucleus, a finding that wa s further supported by cytochemical analysis. Immunofluorescence studi es revealed that nuclear proteins modified by transglutaminase exhibit ed a discrete punctate, as well as a diffuse staining pattern. Further more, different proteins were modified by transglutaminase in the nucl eus compared with the cytosol, The results of these experiments clearl y demonstrate localization of tissue transglutaminase in the nucleus t hat can be activated. These findings may have important implications i n the formation of the insoluble nuclear inclusions, which are charact eristic of codon reiteration diseases such as Huntington's disease and the spinocerebellar ataxias.