The essential Glu124 side chain participates in a network of ionic and hydrogen bond contacts involving the two α-helices and the connecting loop (Figure 6A). Glu124 (in α1) makes a bidentate salt bridge to Arg97 (in α1 and conditionally essential at 18°) and accepts a hydrogen bond from the main-chain amide of Phe108 (in the loop). Arg97, in turn, donates hydrogen bonds to Gln100 and the main-chain carbonyl of Pro106. It was shown previously that replacing Gln110 with arginine results in cold-sensitive and temperature-sensitive growth defects (Tan et al. 2003).