Two-dimensional crystal packing of matrix porin. A channel forming protein in Escherichia coli outer membranes.

Two-dimensional crystalline porin sheets were obtained by reconstitution of monodisperse protein trimers and phospholipids (dimyristoylphosphatidylcholine) by detergent dialysis, analogous to the reconstitution method used for functional tests (Schindler & Rosenbusch, 1981). Three different packing arrangements were observed: two were hexagonal (with p3 symmetry and lattice constants of 9.3 nm and 7.9 nm), and one rectangular (a = 7.9 nm, b = 13.9 nm). The different crystals could be correlated to phospholipid-to-protein weight ratios of 0.16 to 0.72. At the higher ratio, large hexagonal lattices predominated. Higher lipid ratios did not reveal other crystal forms. The packing arrangement of the large hexagonal form appears very similar to the hexagonal habit of three-dimensional crystal forms (Garavito et al., 1983). The shape of the stain-penetrated triplet indentations appeared conserved in the crystal forms to a resolution of 2.2 nm. The mass distribution between triplets, however, were significantly different. They are likely to correspond primarily to lipids. Mass determinations of unstained porin by scanning transmission electron microscopy showed that unit cells consisted of single trimers. The mass found (100,000 daltons) is in good agreement with the value obtained by sedimentation equilibrium analysis.