XAIP1: a Xenopus homologue of yeast actin interacting protein 1 (AIP1), which induces disassembly of actin filaments cooperatively with ADF/cofilin family proteins.

We carried out affinity column chromatography using Xenopus ADF/cofilin (XAC), identified several polypeptides in oocytes specifically bound to this column with actin, and isolated a full-length cDNA clone for a 65 kDa protein in this fraction. The predicted amino acid sequence revealed that the 65 kDa protein has seven obvious WD repeats and exhibits striking homology with yeast actin interacting protein 1 (AIP1). Thus, we designated this protein Xenopus AIP1 (XAIP1). We purified XAIP1 from Xenopus oocytes, and its interaction with actin was characterized by a pelleting assay, photometrical analysis and electron microscopy. Although XAIP1 itself cosedimented with F-actin and increased unsedimented actin to some extent, it induced a rapid, drastic disassembly of actin filaments associated with XAC. Electron microscopic observation revealed that XAIP1 severs actin filaments in the presence of XAC. To elucidate the in vivo effects of XAIP1, the purified protein was injected into blastomeres at the two-cell stage. Although the localization of XAIP1 was similar to that of XAC, at the cortical cytoskeleton and diffusely in the cytoplasm, injection of a large amount of XAIP1 arrested development and abolished the strong cortical staining of both actin and XAC. From these results, we concluded that XAIP1 regulates the dynamics of the cortical actin cytoskeleton cooperatively with XAC in eggs.