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J. Biol. Chem., Vol. 283, Issue 5, 2709-2715, February 1, 2008

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Four-turn -Helical Segment Prevents Surface Expression of the Auxiliary hβ2 Subunit of BK-type Channel^*

Caixia Lv¹, Maorong Chen¹, Geliang Gan¹, Lifen Wang, Tao Xu², and Jiuping Ding³

From the Key Laboratory of Molecular Biophysics (Huazhong University of Science and Technology), Ministry of Education, Wuhan, Hubei 430074, China and the National Laboratory of Biomacromolecules, Institute of Biophysics, Chinese Academy of Science, Beijing 100101, China

Large conductance, voltage- and Ca²⁺-activated K⁺ (BK) channels encoded by the mslo and β2 subunits exist abundantly in rat chromaffin cells, pancreatic β cells, and DRG neurons. The extracellular loop of hβ2 acting as the channel regulator influences the rectification and toxin sensitivity of BK channels, and the inactivation domain at its N terminus induces rapid inactivation. However, the regulatory mechanism, especially the trafficking mechanism of hβ2, is still unknown. With the help of immunofluorescence and patch clamp techniques, we determine that the hβ2 subunit alone resides in the endoplasmic reticulum, suggesting that trafficking mechanism of hβ2 differs from that of hβ1 opposite to what we predicted previously. We further demonstrate that a four-turn helical segment at the N terminus of hβ2 prevents the surface expression of hβ2, that is, the helical segment itself is a retention signal. Using the c-Myc epitope-tagged extracellular loop of hβ2, we reveal that the most accessible site by antibody is located at the middle of the extracellular loop, which might provide clues to understand how the auxiliary β subunits regulates the toxin sensitivity and the rectification of BK-type channels.

Received for publication, May 30, 2007 , and in revised form, October 19, 2007.

^* This work was supported by National Science Foundation of China Grants 30470449, 30470646, 30630020, and 30670502. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ These authors contributed equally to this work.

² To whom correspondence may be addressed. Tel.: 86-10-648-884-69; Fax: 86-10-648-675-66; E-mail: xutao{at}ibp.ac.cn. ³ To whom correspondence may be addressed. Tel.: 86-27-877-921-53; Fax: 86-27-877-920-24; E-mail: jpding{at}mail.hust.edu.cn.

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