Location of an extrinsic label in the primary and tertiary structure of bacteriorhodopsin.

We located a heavy metal label, mercurilated phenylglyoxal, in both the primary sequence and in the tertiary structure of bacteriorhodopsin. This label modified arginines 225 and 227, which are on the COOH-terminal helix (G). In the projected electron potential difference map, the major site is close to the central inner helix. From this result we conclude that helix 1 could not be the COOH-terminal helix G. We tested the multiple isomorphous replacement method for obtaining phases for purple membrane by electron diffraction.