IMMUNO-ATOMIC FORCE MICROSCOPY OF PURPLE MEMBRANE

The atomic force microscope is a useful tool for imaging native biolog ical structures at high resolution. In analogy to conventional immunol abeling techniques, we have used antibodies directed against the C-ter minus of bacteriorhodopsin to distinguish the cytoplasmic and extracel lular surface of purple membrane while imaging in buffer solution. At forces greater than or equal to 0.8 nN the antibodies were removed by the scanning stylus and the molecular topography of the cytoplasmic pu rple membrane surface was revealed. When the stylus was retracted, the scanned membrane area was relabeled with antibodies within 10 min, Th e extracellular surface of purple membrane was imaged at 0.7 nm resolu tion, exhibiting a major and a minor protrusion per bacteriorhodopsin monomer. As confirmed by immune-dot blot analysis and sodium dodecyl s ulfate-gel electrophoresis, labeling of the purple membrane was not ob served if the C-terminus of bacteriorhodopsin was cleaved off by papai n.