ORIENTATION OF PORIN CHANNELS IN THE OUTER-MEMBRANE OF BORDETELLA-PERTUSSIS

We have examined the surface topography and channel connectivity, of a naturally crystalline porin that is known to be functional, and whose structure has not been perturbed by detergent extraction. A three-dim ensional density map, calculated from two independent tilt series of n egatively stained cell envelopes, reveals three separate channels per trimer on one side (the 'smooth' side), and a single common opening at the other ('rough') side. This arrangement is consistent with the mol ecular structures recently determined at high resolution by X-ray crys tallography for three other porins after detergent solubilization, and implies that the Bordetella pertussis porin may have the same kind of folding. Surface relief maps calculated from electron micrographs of cell envelopes contrasted by unidirectional shadowing clearly show tha t the side with single opening (i.e. the rough side) represents the ex ternal surface.