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Journal of Bacteriology, October 2008, p. 6501-6508, Vol. 190, No. 19
0021-9193/08/$08.00+0     doi:10.1128/JB.00665-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

The Helix-Loop-Helix Motif at the N Terminus of HalI Is Essential for Its Immunity Function against Halocin C8 {triangledown} ,{dagger}

Shuangshuang Mei, Chaomin Sun,{ddagger} Xiaoqing Liu, Qiuhe Lu, Lei Cai, Yun Li,§ and Hua Xiang*

State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Sciences, Beijing 100101, People's Republic of China

Received 13 May 2008/ Accepted 21 July 2008

Halocin C8 (HalC8) is a stable microhalocin exhibiting strong antimicrobial activity against a wide range of haloarchaea. HalI, a 207-amino-acid peptide derived from the N terminus of the HalC8 preproprotein, is the immunity protein of HalC8. In this study, the molecular mechanism of the immunity function of HalI was investigated. Both pull-down and surface plasmon resonance assays revealed that HalI directly interacted with HalC8, and a mixture of purified HalI and HalC8 readily formed a heterocomplex, which was verified by gel filtration. Interestingly, HalC8 tended to form a self-associated complex, and one immunity protein likely sequestered multiple halocins. Significantly, the helix-loop-helix (HLH) motif containing a 4-amino-acid repeat (RELA) at the N terminus of HalI played a key role in its immunity activity. Disruption of the HLH motif or mutagenesis of the key residues of the RELA repeat resulted in loss of both the immunity function and the ability of HalI to bind to HalC8. These results demonstrated that HalI sequestered the activity of HalC8 through specific and direct binding.

* Corresponding author. Mailing address: State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Sciences, Datun Road, Chaoyang District, Beijing 100101, People's Republic of China. Phone and fax: (86) 10-6480-7472. E-mail: xiangh{at}sun.im.ac.cn

{triangledown} Published ahead of print on 25 July 2008.

{dagger} Supplemental material for this article may be found at http://jb.asm.org/.

{ddagger} Present address: Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720.

§ Present address: Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas, TX 75390-9148.

Journal of Bacteriology, October 2008, p. 6501-6508, Vol. 190, No. 19
0021-9193/08/$08.00+0     doi:10.1128/JB.00665-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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