3-DIMENSIONAL STRUCTURE OF DIFFERENT AGGREGATES BUILT-UP BY THE S-LAYER PROTEIN OF THERMUS-THERMOPHILUS

The 100-kDa S-layer protein from Thermus thermophilus HB8 is able to f orm three kinds of crystalline aggregates with different morphologies and symmetries when extracted from crude membranes using slightly diff erent extractive procedures. Interestingly, only the structure showing P6 symmetry represents the native S-layer (called S1), while the othe rs show a very different morphology. In this paper, the three dimensio nal structure of each of these crystals revealing their topographic fe atures at 2-3 nm resolution has been analyzed by electron microscopy a nd image processing. The S1 crystals contain an hexagonal center, prob ably build up by six monomers, where most of the protein mass is conce ntrated. These P6 centers account for the width of the native S-layer, being the connective domains of the subunits (two per subunit) locate d at the outermost part of the structure. Such connections radiating f rom different P6 centers are mediated through centers with P3 symmetry . The other two crystals are also consistent with a lattice based on t rimeric subunits probably related to those found in the connections be tween P6 centers of S1. The most conspicuous feature of these two latt er forms is the presence of trimers of channels that traverse the crys tal in the direction perpendicular to the plane of the membrane. Their distinct morphology, quite different from other S-layer assemblies, s hows a surprising adaptive potential of this protein, and suggests int eresting evolutive relationships to other membrane proteins. (C) 1994 Academic Press, Inc.