Charting the surfaces of the purple membrane

The preponderance of structural data of the purple membrane from X-ray diff raction (XRD), electron crystallography (EC), and atomic force microscopy ( AFM) allows us to ask questions about the structure of bacteriorhodopsin it self, as well as about the information derived from the different technique s. The transmembrane helices of bacteriorhodopsin are quite similar in both EC and XRD models. In contrast, the loops at the surfaces of the purple me mbrane show the highest variability between the atomic models, comparable t o the height variance measured by AFM. The excellent agreement of the AFM t opographs with the atomic models from XRD builds confidence in the results. Small technical difficulties in EC lead to poorer resolution of the loop s tructures, although the combination of atomic models with AFM surfaces allo ws clear interpretation of the extent and flexibility of the loop structure s. While XRD remains the premier technique to determine very-high-resolutio n structures, EC offers a method to determine loop structures unhindered by three-dimensional crystal contacts, and AFM provides information about sur face structures and their flexibility under physiological conditions. (C) 1 999 Academic Press.