Internal dynamics of lactose permease.

The transport protein lactose permease was reconstituted in vesicles of dimyristoylphosphatidylcholine, and the internal dynamics were studied by measuring the fluorescence anisotropy decay of the tryptophan residues and of a covalently bound pyrene label. For the tryptophans three relaxation processes and for the pyrene two relaxation processes with relaxation times in the nanosecond range were observed. The slowest process, of approximately 50 ns, is assigned to orientational fluctuations of membrane-spanning helices. When the temperature is decreased below the lipid-phase transition, this relaxation process is slowed down and restricted in amplitude. Because the transport rate is known to also decrease below the phase transition, this observation suggests a coupling between internal dynamics and transport. This coupling is analyzed on the basis of the Kramers relation for chemical reactions.