Fluorescence spectroscopic techniques have been widely applied to the study of soluble proteins over the past 20 yr, and they are being increasingly applied to membrane proteins, including membrane transporters. Some advantages of fluorescence approaches include high sensitivity (so that relatively small amounts of protein are required), commonly available and affordable instrumentation, and the ability to explore several different (but complementary) aspects of membrane protein structure and function. For comprehensive coverage of the principles and practice of modern fluorescence approaches for studying proteins, the reader is referred to the excellent book authored by Lakowicz (1 ).
