Atomic force microscopy of the three-dimensional crystal of membrane protein, OmpC porin

Three-dimensional microcrystals of OmpC osmoporin were air-dried slowly and imaged in air with an atomic force microscope (AFM). The overall structural features in AFM images are in good agreement with the X-ray diffraction data of these OmpC osmoporin crystals: monoclinic P2₁ with the unit cell constants a=117.6 Å, b=110 Å, c=298.4 Å, β=97°. Such a good correspondence between X-ray diffraction and AFM data suggests that the slow and mild air-drying of these crystals did not induce any significant alterations in the crystal lattices as expected upon crystal dehydration. At the (100) crystal face, individual trimeric protein–detergent complexes were resolved. These results show the potential for studying the molecular structure of microcrystals of integral membrane proteins. This study also suggests that the crystal grew in a fashion of rapid two-dimensional expansion along the bc plane followed by a slow deposition along the a axis, perhaps as a rate-limiting nucleation process. Thus, AFM imaging of air-dried crystals would also be of considerable use in the early stages of a project to grow large three-dimensional crystals of membrane proteins suitable for high-resolution X-ray diffraction studies.