The transmembrane protein prestin is crucial to outer hair cell ͑OHC͒ electromotility and contributes to the sensitivity and frequency selectivity of mammalian hearing. The molecular mechanisms of electromotility remain unclear, but prestin is purported to function as both a voltage sensor and a molecular motor. Understanding the role of prestin requires characterizing its organization and behavior in the plasma membrane. Fluorescence recovery after photobleaching ͑FRAP͒ provides a powerful

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... s to quantitatively study molecular diffusion. However, OHCs are inherently fragile ex vivo, and dynamic studies of prestin require model systems, such as human embryonic kidney ͑HEK͒ cells, expressing fluorescently labeled prestin. Utilizing this system, we provide the first direct, quantitative measurement of prestin lateral mobility. The results show remarkably different diffusion behavior for prestin-green fluorescent protein ͑GFP͒ as compared to a control protein, human somatostatin receptor 5 ͑SSTR5͒. Prestin-GFP FRAP experiments reveal immobile fractions approaching 50%, low effective diffusion coefficients, and recovery times slower than those of SSTR5. Secondary bleaching of a region reveals distinctly different diffusion parameters, which we propose reflect the transient confinement of prestin in the HEK cell. Although uncharacterized, intermolecular interactions between prestin and the membrane and/or cytoskeleton may be important for the unique properties of prestin in electromotile OHCs.