[1] Columbia Univ, Coll Phys & Surg, Dept Biochem & Mol Biophys, Howard Hughes Med Inst, New York, NY 10032 USA
[2] Natl Univ Singapore, Inst Mol & Cell Biol, Singapore 117609, Singapore
[3] Columbia Univ, Coll Phys & Surg, Dept Biol, New York, NY 10032 USA