1. Laboratory of Animal Cell Biology, National Institute of Agrobiological Sciences, Ikenodai 2, Tsukuba, Ibaraki 305-0901, Japan
2. Life Science Center, Asahi Techno Glass Co., Gyoda 1-50-1, Funabashi, Chiba 273-0044, Japan
3. Laboratory of New Silk Materials, National Institute of Agrobiological Sciences, Gouda 1-4-8, Okaya, Nagano 394-0021, Japan