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Journal of Virology, January 2008, p. 1011-1020, Vol. 82, No. 2
0022-538X/08/$08.00+0     doi:10.1128/JVI.01156-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

The U95 Protein of Human Herpesvirus 6B Interacts with Human GRIM-19: Silencing of U95 Expression Reduces Viral Load and Abrogates Loss of Mitochondrial Membrane Potential{triangledown}

W. M. Yeo,1 Yuji Isegawa,2 and Vincent T. K. Chow1*

Human Genome Laboratory, Department of Microbiology, Yong Loo Lin School of Medicine, National University of Singapore, Kent Ridge 117597, Singapore,1 Department of Infectious Disease Control, Graduate School of Medicine, Osaka University, Osaka 565-0871, Japan2

Received 28 May 2007/ Accepted 28 September 2007

To better understand the pathogenesis of human herpesvirus 6 (HHV-6), it is important to elucidate the functional aspects of immediate-early (IE) genes at the initial phase of the infection. To study the functional role of the HHV-6B IE gene encoding U95, we generated a U95-Myc fusion protein and screened a pretransformed bone marrow cDNA library for U95-interacting proteins, using yeast-two hybrid analysis. The most frequently appearing U95-interacting protein identified was GRIM-19, which belongs to the family of genes associated with retinoid-interferon mortality and serves as an essential component of the oxidative phosphorylation system. This interaction was verified by both coimmunoprecipitation and confocal microscopic coimmunolocalization. Short-term HHV-6B infection of MT-4 T-lymphocytic cells induced syncytial formation, resulted in decreased mitochondrial membrane potential, and led to progressively pronounced ultrastructural changes, such as mitochondrial swelling, myelin-like figures, and a loss of cristae. Compared to controls, RNA interference against U95 effectively reduced the U95 mRNA copy number and abrogated the loss of mitochondrial membrane potential. Our results indicate that the high affinity between U95 early viral protein and GRIM-19 may be closely linked to the detrimental effect of HHV-6B infection on mitochondria. These findings may explain the alternative cell death mechanism of expiration, as opposed to apoptosis, observed in certain productively HHV-6B-infected cells. The interaction between U95 and GRIM-19 is thus functionally and metabolically significant in HHV-6B-infected cells and may be a means through which HHV-6B modulates cell death signals by interferon and retinoic acid.

* Corresponding author. Mailing address: Human Genome Laboratory, Department of Microbiology, Yong Loo Lin School of Medicine, National University of Singapore, Kent Ridge 117597, Singapore. Phone: (65) 6516-3691. Fax: (65) 6776-6872. E-mail: micctk{at}nus.edu.sg

{triangledown} Published ahead of print on 10 October 2007.

Journal of Virology, January 2008, p. 1011-1020, Vol. 82, No. 2
0022-538X/08/$08.00+0     doi:10.1128/JVI.01156-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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