[1] Univ Toronto, Dept Zool, Toronto, ON M5S 3G5, Canada
[2] Ontario Canc Inst, Div Mol & Struct Biol, Toronto, ON M5G 2M9, Canada
[3] Univ Toronto, Dept Med Biophys, Toronto, ON M5G 2M9, Canada
[4] Univ N Carolina, Dept Biol, Chapel Hill, NC 27599 USA