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Removal of a putative inhibitory element reduces the calcium-dependent calmodulin activation of neuronal nitric-oxide
H J Montgomery1, V Romanov, J G Guillemette
1Department of Chemistry and the Guelph Waterloo Centre for Graduate Work in Chemistry and Biochemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
Abstract:
Neuronal nitric-oxide synthase (NOS) and endothelial NOS are constitutive NOS isoforms that are activated by binding calmodulin in response to elevated intracellular calcium. In contrast, the inducible NOS isoform binds calmodulin at low basal levels of calcium in resting cells. Primary sequence comparisons show that each constitutive NOS isozyme contains a polypeptide segment within its reductase domain, which is absent in the inducible NOS enzyme. To study a possible link between the presence of these additional polypeptide segments in constitutive NOS enzymes and their calcium-dependent calmodulin activation, three deletion mutants were created. The putative inhibitory insert was removed from the FMN binding regions of the neuronal NOS holoenzyme and from two truncated neuronal NOS reductase enzymes in which the calmodulin binding region was either included or deleted. All three mutant enzymes showed reduced incorporation of FMN and required reconstitution with exogenous FMN for activity. The combined removal of both the calmodulin binding domain and the putative inhibitory insert did not result in a calmodulin-independent neuronal NOS reductase. Thus, although the putative inhibitory element has an effect on the calcium-dependent calmodulin activation of neuronal NOS, it does not have the properties of the typical autoinhibitory domain found in calmodulin-activated enzymes.
Insights
Researchers investigated the role of a specific protein segment in neuronal nitric-oxide synthase (NOS) calcium-dependent activation. Deleting this segment affected enzyme activity but did not lead to calmodulin-independent function, suggesting it
Area of Science:
- Biochemistry
- Molecular Biology
- Enzymology
Background:
- Constitutive nitric-oxide synthase (NOS) isoforms, like neuronal NOS and endothelial NOS, are activated by calcium-bound calmodulin.
- Inducible NOS (iNOS) binds calmodulin at low calcium levels.
- Constitutive NOS isoforms possess a unique polypeptide segment in their reductase domain absent in iNOS.
Purpose of the Study:
- To investigate the link between the unique polypeptide segment in constitutive NOS and calcium-dependent calmodulin activation.
- To determine if this segment acts as a typical autoinhibitory domain.
Main Methods:
- Creation of three deletion mutants of neuronal NOS.
- Removal of the putative inhibitory insert from FMN binding regions.
- Analysis of mutants with and without the calmodulin binding domain.
Main Results:
- All three mutant enzymes exhibited reduced FMN incorporation, necessitating exogenous FMN for activity.
- Combined deletion of the calmodulin binding domain and the putative inhibitory insert did not yield a calmodulin-independent neuronal NOS reductase.
- The putative inhibitory element influences calcium-dependent calmodulin activation but lacks typical autoinhibitory domain properties.
Conclusions:
- The specific polypeptide segment in neuronal NOS affects calcium-dependent calmodulin activation.
- This segment does not function as a canonical autoinhibitory domain in calmodulin-activated enzymes.
- Further research is needed to fully elucidate the regulatory mechanisms of NOS isoforms.