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J. Biol. Chem., Vol. 283, Issue 24, 16915-16927, June 13, 2008

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Afi1p Functions as an Arf3p Polarization-specific Docking Factor for Development of Polarity^*

Pei-Chin Tsai¹, Szu-Wei Lee¹, Ya-Wen Liu¹, Chih-Wen Chu, Kuan-Yu Chen, Jui-Chih Ho, and Fang-Jen S. Lee²

From the Institute of Molecular Medicine, School of Medicine, National Taiwan University, and the Department of Medical Research, National Taiwan University Hospital, Taipei 100, Taiwan

ADP-ribosylation factors (Arfs) are highly conserved small GTPases and are critical components of vesicle trafficking. Yeast Arf3p, despite its similarity to mammalian Arf6, is not required for endocytosis but is involved in polarity development. In this study, we identified an Arf3p interacting protein 1 (Afi1p), which, through its N-terminal conserved region, specifically interacts with GTP-bound Arf3p. Afi1p is distributed asymmetrically at the plasma membrane and is required for polarized distribution of Arf3p but not of an Arf3p guanine nucleotide-exchange factor, Yel1p. However, Afi1p is not required for targeting of Arf3p or Yel1p to the plasma membrane. Like arf3 mutant yeast, afi1 mutant yeast exhibited an abnormal budding pattern and partially delayed actin patch polarization. An Afi1p, ³⁸KLGP4A-Afi1p, mutated at the Arf3p-binding region, loses its ability to interact with Arf3p and maintain the polarized distribution of Arf3p. Although ³⁸KLGP4A-Afi1p still possessed a proper polarized distribution, it lost its ability to rescue actin patch polarization in afi1 mutant cells. Our findings demonstrate that Afi1p functions as an Arf3p polarization-specific adapter and participates in development of polarity.

Received for publication, April 2, 2008

^* This work was supported by National Science Council Grant NSC-90-2320-B-002-003; the Program for Promoting Academic Excellence of University Grants NSC-93-2752-B-002-007-PAE, NSC-94-2752-B-002-007-PAE, and NSC-95-2752-B-002-007-PAE; and the Yung-Shin Biomedical Research Funds YSP-86-019 (to F.-J. S. L.). 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1-S4.

¹ These authors contributed equally to this work.

² To whom correspondence should be addressed: National Taiwan University Hospital, 7 Chung Shan SS. Rd., Taipei 100, Taiwan. Tel.: 886-2-2312-3456 (Ext. 5730); Fax: 886-2-2395-7801; E-mail: fangjen{at}ntu.edu.tw.

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