Location of platinum binding sites on bacteriorhodopsin by electron diffraction

Abstract

A Pt-containing derivative of Halobacterium halobium bacteriorhodopsin was prepared by treating purple membranes with glycyl-L-methionatoplatinum. Low-dose electron diffraction was used to identify Pt binding sites in the 5.6 .ANG. resolution reconstruction of the bacteriorhodopsin unit cell in projection. This is a necessary 1st step in the use of the Pt derivative for identifying the parts of the amino acid sequence corresponding to the .alpha.-helices in the bacteriorhodopsin structure and for obtaining phases for reflections out to 3.5 .ANG. resolution by the method of heavy-atom isomorphous replacement. The largest peak in a Fourier difference map between Pt-labeled and native purple membrane is larger than the spurious features expected to arise from errors in measurements of diffraction intensities.