Crystal structures of the signal transducing protein GlnK from Thermus thermophilus HB8

The Thermus thermophilus HB8 genome encodes a signal transducing PII protein, GlnK. The crystal structures of GlnK have been determined in two different space groups, P212121 and P3121. The PII protein has the T-loop, which is essential for interactions with receptor proteins. In both crystal forms, three GlnK molecules form a trimer in the asymmetric unit. In one P212121 crystal form, the three T-loops in the trimer are disordered, while in another P212121 crystal form, the T-loop from one molecule in the trimer is ordered. In the P3121 crystal, one T-loop is ordered while the other two T-loops are disordered. The conformations of the ordered T-loops significantly differ between the two crystal forms; one makes the α-helix in the middle of the T-loop, while the other has an extension of the β-hairpin. Two different conformations are captured by the crystal contacts. The observation of multiple T-loop conformations suggests that the T-loop could potentially exhibit “polysterism,” which would be important for interactions with receptor proteins. The crystal structures of the nucleotide-bound forms, GlnK·ATP and GlnK·ADP, have also been determined. ATP/ADP binding within a cleft at the interface of two adjacent T. thermophilus GlnK monomers might affect the conformation of the T-loop.