SNTN, also known as sentan, belongs to the S-100 family. SNTN is shown to localize exclusively to the bridging structure between the cell membrane and peripheral singlet microtubules, which specifically exists in the narrowed distal portion of cilia. Exogenously expressed sentan showed affinity for the membrane protrusions, and a protein-lipid binding assay revealed that sentan bound to phosphatidylserine. These findings suggest that sentan is the first molecular component of the ciliary tip to bridge the cell membrane and peripheral singlet microtubules, making the distal portion of the cilia narrow and stiff to allow for better airway clearance or ovum transport. Recombinant human SNTN protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography.