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Biphasic control logic of HAMP domain signalling in the Escherichia coli serine chemoreceptor.
Mol Microbiol. 2011 May; 80(3):596-611
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19 Apr 2011
New Finding
DOI: 10.3410/f.9783967.10482067
In this remarkable paper, which is a tour-de-force illustrating the power of genetics, the Parkinson lab has shown that the HAMP domain, which is in the cytoplasm adjacent to the plasma membrane and transduces the effects of attractant or repellent binding to the receptor outside of the membrane, can...Continue reading
In this remarkable paper, which is a tour-de-force illustrating the power of genetics, the Parkinson lab has shown that the HAMP domain, which is in the cytoplasm adjacent to the plasma membrane and transduces the effects of attractant or repellent binding to the receptor outside of the membrane, can be in three possible states, not just two, as has been generally supposed. This hypothesis allows us to understand a number of paradoxical mutants and lays an important foundation for a realistic understanding of how the receptors control the kinase.
'Below' the HAMP domain is the methylation domain, where receptor methylation or demethylation can reverse the effect of attractant addition or removal. Attractants inactivate the kinase to cause smooth swimming and repellents activate it to cause tumbling in wild type. There is a 'stutter' ('out of register' region) between the helices of the parallel four helix bundle of the HAMP domain and those of the antiparallel four helix bundle of the methylation helix 'below' it. The CheA kinase binds even farther down, where the helical dimer turns back on itself in creating the four helix bundle. The presumption has been that increased stability of the HAMP domain, as caused by attractants, leads to less stability of the methylation helices, leading to decreased kinase activity (resulting in smooth swimming). Methylation as an adaptation process increases the stability of the methylation helices and, thus, restores kinase activity.
These investigators have created 118 mutants in the HAMP domain and, sure enough, they have created mutants where, in the right enzymatic background, the HAMP domain is very stable compared to the wild-type receptor and there is little kinase activity as the result. However, they have also created mutants where the HAMP domain is very unstable and mutants where receptor deamidation (equivalent to demethylation) produces increased kinase activity (resulting in tumbling), as well as where methylation catalyzed by the CheR methyltransferase leads to decreased kinase activity (resulting in increased smooth swimming). In the wild-type receptor, as mentioned, methylation increases kinase activity and demethylation (deamidation) decreases it. The only way to account for such phenomena is to assume that both a very stable HAMP domain and a very unstable HAMP domain lead to decreased kinase activity, but for very different structural reasons. Of course, the very unstable HAMP domain is not observable except in mutant strains but the fact that it can exist lays the foundation for a more realistic account of the types of changes in the receptor that control the kinase.
'Below' the HAMP domain is the methylation domain, where receptor methylation or demethylation can reverse the effect of attractant addition or removal. Attractants inactivate the kinase to cause smooth swimming and repellents activate it to cause tumbling in wild type. There is a 'stutter' ('out of register' region) between the helices of the parallel four helix bundle of the HAMP domain and those of the antiparallel four helix bundle of the methylation helix 'below' it. The CheA kinase binds even farther down, where the helical dimer turns back on itself in creating the four helix bundle. The presumption has been that increased stability of the HAMP domain, as caused by attractants, leads to less stability of the methylation helices, leading to decreased kinase activity (resulting in smooth swimming). Methylation as an adaptation process increases the stability of the methylation helices and, thus, restores kinase activity.
These investigators have created 118 mutants in the HAMP domain and, sure enough, they have created mutants where, in the right enzymatic background, the HAMP domain is very stable compared to the wild-type receptor and there is little kinase activity as the result. However, they have also created mutants where the HAMP domain is very unstable and mutants where receptor deamidation (equivalent to demethylation) produces increased kinase activity (resulting in tumbling), as well as where methylation catalyzed by the CheR methyltransferase leads to decreased kinase activity (resulting in increased smooth swimming). In the wild-type receptor, as mentioned, methylation increases kinase activity and demethylation (deamidation) decreases it. The only way to account for such phenomena is to assume that both a very stable HAMP domain and a very unstable HAMP domain lead to decreased kinase activity, but for very different structural reasons. Of course, the very unstable HAMP domain is not observable except in mutant strains but the fact that it can exist lays the foundation for a more realistic account of the types of changes in the receptor that control the kinase.
Disclosures
None declared
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Ordal G W: F1000Prime Recommendation of [Zhou Q et al., Mol Microbiol 2011, 80(3):596-611]. In F1000Prime, 19 Apr 2011; DOI: 10.3410/f.9783967.10482067. F1000Prime.com/9783967#eval10482067
F1000Prime Recommendations, Dissents and Comments for [Zhou Q et al., Mol Microbiol 2011, 80(3):596-611]. In F1000Prime, 18 May 2013; F1000Prime.com/9783967
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HAMP domains mediate input-output communication in many bacterial signalling proteins. To explore the dynamic bundle model of HAMP signalling (Zhou et al., Mol. Microbiol. 73: 801, 2009), we characterized the signal outputs of 118 HAMP missense mutants of the serine chemoreceptor, Tsr, by flagellar rotation patterns. Receptors with proline or charged amino acid replacements at critical hydrophobic packing residues in the AS1 and AS2 HAMP helices had locked kinase-off outputs, indicating that drastic destabilization of the Tsr-HAMP bundle prevents kinase activation, both in the absence and presence of the sensory adaptation enzymes, CheB and CheR. Attractant-mimic lesions that enhance the structural stability of the HAMP bundle also suppressed kinase activity, demonstrating that Tsr-HAMP has two kinase-off output states at opposite extremes of its stability range. HAMP mutants with locked-on kinase outputs appeared to have intermediate bundle stabilities, implying a biphasic relationship between HAMP stability and kinase activity. Some Tsr-HAMP mutant receptors exhibited reversed output responses to CheB and CheR action that are readily explained by a biphasic control logic. The findings of this study provide strong support for a three-state dynamic bundle model of HAMP signalling in Tsr, and possibly in other bacterial transducers as well.
© 2011 Blackwell Publishing Ltd.
DOI: 10.1111/j.1365-2958.2011.07577.x
PMID: 21306449
