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J. Cell Biol.,
Volume 144, Number 5, March 8, 1999 1001-1018
* Takai Biotimer Project, ERATO, Japan Science and Technology Corporation, c/o JCR Pharmaceuticals Co., Ltd., 2-2-10 Murotani, Nishi-ku, Kobe 651-2241, Japan; We recently isolated a novel actin filament
(F-actin)-binding protein, afadin, that has two isoforms, l- and s-afadins. l-Afadin is ubiquitously expressed and specifically localized at zonula adherens
(ZA) in epithelial cells and at cell-cell adherens junction (AJ) in nonepithelial cells, whereas s-afadin is
abundantly expressed in neural tissue. l-Afadin has
one PDZ domain, three proline-rich regions, and one
F-actin-binding domain, whereas s-afadin lacks the
third proline-rich region and the F-actin-binding domain. To understand the molecular mechanism of the
specific localization of l-afadin at ZA in epithelial cells
and at cell-cell AJ in nonepithelial cells, we attempted
here to identify an l-afadin-binding protein(s) and isolated a protein, named ponsin. Ponsin had many splicing variants and the primary structures of two of them
were determined. Both the two variants had three Src
homology 3 (SH3) domains and turned out to be splicing variants of SH3P12. The third proline-rich region of
l-afadin bound to the region of ponsin containing the second and third SH3 domains. Ponsin was ubiquitously
expressed and localized at ZA in epithelial cells, at
cell-cell AJ in nonepithelial cells, and at cell-matrix AJ
in both types of cells. Ponsin furthermore directly bound
vinculin, an F-actin-binding protein localized at ZA in
epithelial cells, at cell-cell AJ in nonepithelial cells, and
at cell-matrix AJ in both types of cells. Vinculin has one proline-rich region where two proline-rich sequences
are located. The proline-rich region bound to the region
of ponsin containing the first and second SH3 domains.
l-Afadin and vinculin bound to ponsin in a competitive
manner and these three proteins hardly formed a ternary complex. These results indicate that ponsin is an
l-afadin- and vinculin-binding protein localized at ZA
in epithelial cells, at cell-cell AJ in nonepithelial cells,
and at cell-matrix AJ in both types of cells.
Department of Anatomy and Neurobiology, Faculty of Medicine, Kyoto University,
Kyoto 606-8501, Japan; and § Department of Molecular Biology and Biochemistry, Osaka University Medical School, Suita
565-0871, Japan
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