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 (XA C), identified several polypeptides in oocytes specifically bound to this c olumn with actin, and isolated a full-length cDNA clone for a 65 kDa protei n in this fraction, The predicted amino acid sequence revealed that the 65 kDa protein has seven obvious WD repeats and exhibits striking homology wit h yeast actin interacting protein 1 (AIP1). Thus, we designated this protei n Xenopus AIP1 (XAIP1). We purified XAIP1 from Xenopus oocytes, and its int eraction with actin was characterized by a pelleting assay, photometrical a nalysis 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 micros copic observation revealed that XAIP1 severs actin filaments in the presenc e of XAC, To elucidate the in vivo effects of XAIP1, the purified protein w as injected into blastomeres at the two-cell stage. Although the localizati on of XAIP1 was similar to that of XAC, at the cortical cytoskeleton and di ffusely in the cytoplasm, injection of a large amount of XAIP1 arrested dev elopment and abolished the strong cortical staining of both actin and XAC. From these results, we concluded that XAIP1 regulates the dynamics of the c ortical actin cytoskeleton cooperatively with XAC in eggs.