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Molecular and Cellular Biology, April 2002, p. 2810-2820, Vol. 22, No. 8
0270-7306/02/$04.00+0     DOI: 10.1128/MCB.22.8.2810-2820.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

HMG2 Interacts with the Nucleosome Assembly Protein SET and Is a Target of the Cytotoxic T-Lymphocyte Protease Granzyme A

Zusen Fan, Paul J. Beresford, Dong Zhang, and Judy Lieberman*

Center for Blood Research and Department of Pediatrics, Harvard Medical School, Boston, Massachusetts 02115

Received 28 December 2001/ Accepted 22 January 2002

The cytotoxic T-lymphocyte protease granzyme A induces caspase-independent cell death in which DNA single-stranded nicking is observed instead of oligonucleosomal fragmentation. A 270- to 420-kDa endoplasmic reticulum-associated complex (SET complex) containing the nucleosome assembly protein SET, the tumor suppressor pp32, and the base excision repair enzyme APE can induce single-stranded DNA damage in isolated nuclei in a granzyme A-dependent manner. The normal functions of the SET complex are unknown, but the functions of its components suggest that it is involved in activating transcription and DNA repair. We now find that the SET complex contains DNA binding and bending activities mediated by the chromatin-associated protein HMG2. HMG2 facilitates assembly of nucleoprotein higher-order structures by bending and looping DNA or by stabilizing underwound DNA. HMG2 is in the SET complex and coprecipitates with SET. By confocal microscopy, it is observed that cytoplasmic HMG2 colocalizes with SET in association with the endoplasmic reticulum, but most nuclear HMG2 is unassociated with SET. This physical association suggests that HMG2 may facilitate the nucleosome assembly, transcriptional activation, and DNA repair functions of SET and/or APE. HMG2, like SET and APE, is a physiologically relevant granzyme A substrate in targeted cells. HMG1, however, is not a substrate. Granzyme A cleavage after Lys65 in the midst of HMG box A destroys HMG2-mediated DNA binding and bending functions. Granzyme A cleavage and functional disruption of key nuclear substrates, including HMG2, SET, APE, lamins, and histones, are likely to cripple the cellular repair response to promote cell death in this novel caspase-independent death pathway.

* Corresponding author. Mailing address: The Center for Blood Research, 800 Huntington Ave., Boston, MA 02115. Phone: (617) 278-3381. Fax: (617) 278-3493. E-mail: lieberman{at}cbr.med.harvard.edu.

Molecular and Cellular Biology, April 2002, p. 2810-2820, Vol. 22, No. 8
0022-538X/02/$04.00+0     DOI: 10.1128/MCB.22.8.2810-2820.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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