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Nat Chem Biol. 2013 Jul;9(7):437-43. doi: 10.1038/nchembio.1252. Epub 2013 May 12.

Chemically inducible diffusion trap at cilia reveals molecular sieve-like barrier.

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Department of Cell Biology, Center for Cell Dynamics, School of Medicine, Johns Hopkins University.
Departments of Medicine and Biochemistry, School of Medicine, Stanford University.
Department of Biomedical Engineering, Johns Hopkins University.
Department of Life Science and Medical Bioscience, School of Advanced Science and Engineering, Waseda University.
PRESTO Investigator, JST, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan.
Contributed equally


Primary cilia function as specialized compartments for signal transduction. The stereotyped structure and signaling function of cilia inextricably depend on the selective segregation of molecules in cilia. However, the fundamental principles governing the access of soluble proteins to primary cilia remain unresolved. We developed a methodology termed 'chemically inducible diffusion trap at cilia' to visualize the diffusion process of a series of fluorescent proteins ranging in size from 3.2 nm to 7.9 nm into primary cilia. We found that the interior of the cilium was accessible to proteins as large as 7.9 nm. The kinetics of ciliary accumulation of this panel of proteins was exponentially limited by their Stokes radii. Quantitative modeling suggests that the diffusion barrier operates as a molecular sieve at the base of cilia. Our study presents a set of powerful, generally applicable tools for the quantitative monitoring of ciliary protein diffusion under both physiological and pathological conditions.

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