Neuronal nitric oxide synthase (nNOS) in fast-twitch skeletal muscle fibers is primarily particulate in contrast to its greater solubility in brain. Immunohistochemistry shows nNOS localized to the sarcolemma, with enrichment at force transmitting sites, the myotendinous junctions, and costameres. Because this distribution is similar to dystrophin, we determined if nNOS expression was affected by the loss of dystrophin. Significant nNOS immunoreactivity and enzyme activity was absent in skeletal muscle tissues from patients with Duchenne muscular dystrophy. Similarly, in dystrophin-deficient skeletal muscles from mdx mice both soluble and particulate nNOS was greatly reduced compared with C57 control mice. nNOS mRNA was also reduced in mdx muscle in contrast to mRNA levels for a dystrophin binding protein, cal-syntrophin. nNOS levels increased dramatically from 2 to 52 weeks of age in C57 skeletal muscle, which may indicate a physiological role for NO in aging-related processes. Biochemical purification readily dissociates nNOS from the dystrophin-glycoprotein complex. Thus, nNOS is not an integral component of the dystrophin-glycoprotein complex and is not simply another dystrophin-associated protein since the expression of both nNOS mRNA and protein is affected by dystrophin expression.

Functional inactivation of p53 and constitutive activation of the NF-B pathway has been associated with several human cancers. In this study, we show that I B kinase 2 (IKK2/IKK␤), which is critical for NF-B activation, also phosphorylates p53. Phosphorylation of p53 at serines 362 and 366 by IKK2 leads to its recruitment to and ubiquitination by ␤-TrCP1. Degradation of ubiquitinated p53 is independent of Mdm2, because it occurs in both wild-type and Mdm2 ؊/؊ cells. SiRNA-mediated reduction in the levels of ␤-TrCP1 and other members of the SCF ␤؊TrCP1 E3 ubiquitin ligase complex or overexpression of a dominant negative form of ␤-TrCP1 enhances p53 stability. Substitutions at Ser-362 and 366 of p53 by alanines (p53 AA) result in reduced phosphorylation of p53 by IKK2, decreased association with ␤-TrCP1, and thus increased stability of p53 and expression of p53 target genes such as p21, altering the G1 phase of the cell cycle. Our results identify IKK2 and ␤-TrCP1 as novel regulators of the p53 pathway and suggest that blocking of IKK2 and ␤-TrCP1 could be a means of regulating p53 stability and thereby modulating its biological activity.␤-TrCP1 ͉ DNA damage ͉ IKK2 ͉ p53 stability N F-B is a family of transcription factors crucial for several biological processes involved in innate and adaptive immunity, inflammation, cell survival, and cancer (1). More than 150 different types of stimuli, including proinflammatory cytokines, bacterial/ viral infection, and stress, activate NF-B-dependent transcription of target genes. Activation of the I B kinase (IKK) complex is a prerequisite for NF-B activation. Once activated, IKK phosphorylates serines 32 and 36 of I B proteins (2), followed by ubiquitination by ␤-TrCP1, an E3 ubiquitin ligase, which targets them for proteosomal degradation (3). IKK2 has been shown to phosphorylate other cellular and viral proteins, such as ␤-catenin, I B␤, I B , p65, HIV-VPU, and SRC-3; this phosphorylation regulates the steady-state levels and transcriptional activity of these substrates (4-7). Approximately 30% of cellular proteins contain covalently bound phosphate, even though only 500-600 protein kinases likely are encoded by the human genome (8). Therefore, it is conceivable that, like many other kinases, IKK2 also has additional substrates, and that each protein phosphorylated by IKK2 in turn can be a substrate of other kinases.The p53 tumor suppressor plays a crucial role in the development of many cancers and is frequently mutated or deleted in human cancers (9, 10). In the absence of stress, endogenous p53 levels are very low, because of the constant recruitment of p53 to Mdm2, an E3 ubiquitin ligase that inhibits its transcriptional activity and targets it for proteosomal degradation (11,12). In response to DNA damage and other cellular stresses, the p53 protein levels are up-regulated and its activities are induced. The inhibition of Mdm2 binding to p53 and increases in p53 transcriptional activity involve the phosphorylation and stabilization of p53.Human p53 has been reported ...

Ca2+/calmodulin activates myosin light chain kinase by reversal of an autoinhibited state. The effects of substitution mutations on calmodulin activation properties implicate 4 of the 8 basic residues between the catalytic core and the calmodulin-binding domain in maintaining autoinhibition. These residues are further amino-terminal to the basic residues comprising the previously proposed pseudosubstrate sequence and suggest involvement of the connecting region in intrasteric autoinhibition. The pseudosubstrate model for autoinhibition proposes that basic residues within the autoinhibitory region mimic basic residues in the substrate and bind to defined acidic residues within the catalytic core. Charge reversal mutations of these specific acidic residues, however, had little or no effect on the Km value for regulatory light chain. From a total of 20 acidic residues on the surface of the substrate binding lobe of the catalytic core, 7 are implicated in binding directly or indirectly to the autoinhibitory domain but not to the light chain. Only 2 acidic residues near the catalytic site may bind to the autoinhibitory domain and the arginine at P-3 in the light chain. Exposure of these 2 residues upon calmodulin binding may be necessary and sufficient for light chain phosphorylation.

In skeletal muscle, neuronal nitric oxide synthase is localized at the sarcolemma in association with the dystrophin glycoprotein complex (DGC). The nNOS N-terminal 231 amino acids comprise a PDZ domain (residues 1^100) and a L L-hairpin finger loop (residues 101^130) which binds K K-syntrophin located in the DGC. Endogenous nNOS and GFP-tagged nNOS localize to the sarcolemma in mouse C2C12 myotubes. Expression of GFP-tagged nNOS domains in C2C12 myotubes reveals that the PDZ domain and the L L-hairpin finger loop of nNOS are independently capable of localizing to the sarcolemma of C2C12 myotubes. Binding studies indicate that K K-syntrophin binds only to the L L-hairpin finger loop and not the PDZ domain of nNOS. nNOS may bind to proteins in addition to K K-syntrophin at muscle sarcolemma. ß 2000 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.

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