Packing contacts are crystal artifacts, yet they make use of the same forces that govern specific recognition in protein-protein complexes and oligomeric proteins. They provide examples of a nonspecific protein-protein interaction which can be compared to biologically relevant ones. We evaluate the number and size of pairwise interfaces in 152 crystal forms where the asymmetric unit contains a monomeric protein. In those crystal forms that have no element of 2-fold symmetry, we find that molecules form 8 to 10 pairwise interfaces. The total area of the surface buried on each molecule is large, up to 4400 Å2. Pairwise interfaces bury 200–1200 Å2, like interfaces generated at random in a computer simulation, and less than interfaces in protease-inhibitor or antigen-antibody complexes, which bury 1500 Å2 or more. Thus, specific contacts occurring in such complexes extend over a larger surface than nonspecific ones. In crystal forms with 2-fold symmetry, pairwise interfaces are fewer and larger on average than in the absence of 2-fold symmetry. Some bury 1500–2500 Å2, like interfaces in oligomeric proteins, and create “crystal oligomers” which may have formed in the solution before crystallizing. © 1995 Wiley-Liss, Inc.
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