Differential domain accessibility to monoclonal antibodies in three different morphological assemblies built up by the S-layer protein of Thermus thermophilus HB8

A collection of 27 monoclonal antibodies (MABs) against the S-layer protein (P100) of Thermus thermophilus HB8 has been obtained. They have been classified according to their ability to recognize S-layer regions expressed in E. coli from plasmids containing different fragments of its coding gene, slpA. The accessibility of the binding sites in hexagonal, trigona., or tetragonal assemblies of P100 was analyzed by enzyme-linked immunosorbent assays with six of these MAbs and their respective Fab fragments. When packed hexagonally as the native S-layer (S1 assemblies), only a small region located near the amino terminus of the P100 was accessible. However, when P100 was assembled into trigonal (pS2 assemblies) or tetragonal (S2 assemblies) arrays, most of the protein domains analyzed were easily detected, thus suggesting that P100 is assembled in S2 and pS2 in a similar way and that these two arrangements are quite different from the S1 assembly. Relationships between accessibility and sequence predictions are discussed.