An introduction to membrane proteins.

α-Helical membrane proteins are important for many biological functions. Due to physicochemical constraints, the structures of membrane proteins differ from the structure of soluble proteins. Historically, membrane protein structures were assumed to be more or less two-dimensional, consisting of long, straight, membrane-spanning parallel helices packed against each other. However, during the past decade, a number of the new membrane protein structures cast doubt on this notion. Today, it is evident that the structures of many membrane proteins are equally complex as for many soluble proteins. Here, we review this development and discuss the consequences for our understanding of membrane protein biogenesis, folding, evolution, and bioinformatics.