EIIA was overexpressed, purified, crystallized and the structure solved as described above. The final model contains three dimers of E. faecalis EIIAgnt, covering residues 1–140 (chains B, D, E, F; chain A: 1–137, chain C: 1–139), 10 Ca2+ ions derived from the buffer and 149 water molecules. Residues 141–150 appear to be flexible in the crystals. EIIAgnt is present as a tightly interacting dimer in the crystal structure and consists of a four-stranded parallel β-sheet which is flanked by three helices on each side. In the dimer, both C-termini extend on top of the neighboring monomer and provide an additional strand to the central β-sheet in an antiparallel fashion ( Fig. 1a), ( Table 1). The interface between the monomers covers a surface area of about 1825 ?2 (24%), consists of 50 rather
JNK-IN-8 residues and has 15 hydrogen bonds [25] which is in good accordance with average values for protein interfaces [33]. Considering size and nature of the protein–protein interface we suggest
endoplasmic reticulum (ER) the observed homodimer is physiologically relevant, which was indicated for several other EIIA components as well [1], [6], [34] and [35]. The peptide backbone of EIIAgnt clearly resembles the typical fold of the mannose family EIIA components [2] and [15] and superposition with EIIAman from E. coli [6], [8] and [20] results in a root mean square deviation (RMSD) of 1.6 ? over 133 corresponding residues [24] while the sequence identity is only 24% [36]. Also the EIIAman active site residue His-10 which first receives and then donates the phosphoryl group during the transfer reactions [37] has with His-9 its counterpart in EIIAgnt ( Fig. 2).
