Signaling at the Cytoplasmic Domain

In a mutant of the E. coli serine receptor (Tsr), Q mutant, all four methylation site residues of the Tsr cytoplasmic domain are glutamines, thus mimicking the fully methylated state of the receptor. The phosphorylation activity of this mutant receptor is very high [43]. The Q mutant effectively locks the receptor in a signal "on" state. In contrast to the ligand binding domains, crystal structures of the cytoplasmic domains of the Q mutant [27] and the wild-type receptor [44] show no conformational differences. The crystallographic data described in Fig. 4 [44] revealed the presence of significant differences in dynamic flexibilities of the domains, however, suggesting that modulation of or changes in the dynamic properties of the receptor may be the language in which the signal is transmitted from the chemotaxis receptor to CheA, its immediate downstream effector. The structural basis for the reduced dynamic flexibility in the Q mutant is probably the

Figure 4 Cyrstallographic B factors of a molecule reflect the sum of two properties: dynamic flexibility and static disorder of the molecule. The ABs are calculated using the average B factors of the entire molecule. Q160, all Q mutant at160 K; Q100, all Q mutant at 100 K; W160, wild type at 160 K; W100, wild-type at 100 K. The B factors of either the wild type or Q mutant change little between temperatures 60 K apart; however, the wild type displayed a much higher temperature factor when compared with the Q mutant at both temperatures.

Figure 4 Cyrstallographic B factors of a molecule reflect the sum of two properties: dynamic flexibility and static disorder of the molecule. The ABs are calculated using the average B factors of the entire molecule. Q160, all Q mutant at160 K; Q100, all Q mutant at 100 K; W160, wild type at 160 K; W100, wild-type at 100 K. The B factors of either the wild type or Q mutant change little between temperatures 60 K apart; however, the wild type displayed a much higher temperature factor when compared with the Q mutant at both temperatures.

extensive inter-helical hydrogen bonding formed by the gluta-mine residues at the four methylation sites.

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