The molecular understanding of secondary transport, in particular how transport activity is regulated, is one of the cutting-edge questions in biological science (1). A number of secondary transporters show regulation of transport activity, which plays an important role in nearly all stress-induced cellular responses. Transport activity can be regulated in response to various external stimuli, which often are difficult to identify. Therefore, only a few regulated transporters are described to date; one of the best characterized amongst them is the Na+-coupled betaine symporter BetP from Corynebacterium glutamicum (2). In the last five years several atomic structures of trimeric BetP were solved in different conformational states and under active and inactive conditions (3). First insights into a molecular mechanism of regulated transport in BetP were obtained by the combination of X-ray and electron crystallography data, functional measurements, spectroscopy and bioinformatics. Thereby, the transport cycle of BetP was described in molecular detail. The key towards crystallizing different conformational state was found in a combination of rational mutagenesis of residues involved in substrate-coupling and purification methods that consider strongly the stabilizing effects of lipids.