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We have investigated the mechanisms that target oxidized calmodulin for degradation by the proteasome. After methionine oxidation within calmodulin, rates of degradation by the 20 S proteasome are substantially enhanced. Mass spectrometry was used to identify the time course of the proteolytic fragments released from the proteasome. Oxidized calmodulin is initially degraded into large proteolytic fragments that are released from the proteasome and subsequently degraded into small peptides that vary in size from 6 to 12 amino acids. To investigate the molecular determinants that result in the selective degradation of oxidized calmodulin, we used circular dichroism and fluorescence spectroscopy to assess oxidant-induced structural changes. There is a linear correlation between decreases in secondary structure and the rate of degradation. Calcium binding or the repair of oxidized calmodulin by methionine sulfoxide reductase induces comparable changes in ␣-helical content and rates of degradation. In contrast, alterations in the surface hydrophobicity of oxidized calmodulin do not alter the rate of degradation by the proteasome, indicating that changes in surface hydrophobicity do not necessarily lead to enhanced proteolytic susceptibility. These results suggest that decreases in secondary structure expose proteolytically sensitive sites in oxidized calmodulin that are cleaved by the proteasome in a nonprocessive manner.Critical to the ability of a cell to reestablish cellular homeostasis after a range of different environmental stresses is the removal of oxidatively modified proteins by the proteasome (1, 2). The proteasome represents approximately 1% of the total cellular protein and is present in the cytosol and nuclei of all mammalian cells in two major forms (i.e. the 20 S and 26 S proteasomes) (3). The 20 S proteasome is a 700-kDa complex with a 10 -20-Å-diameter opening into an internal cavity that provides a sequestered environment for proteolysis. The 26 S proteasome is a 2000-kDa complex containing two 19 S regulatory complexes bound to the 20 S multicatalytic core. The 26 S proteasome is responsible for the degradation of the majority of cellular proteins through an ATP-dependent and ubiquitinmediated pathway (4 -6). In contrast, the 20 S proteasome core selectively degrades a range of different oxidized proteins in an ATP-independent manner and has been suggested to represent the primary mechanism in the rapid removal of oxidized proteins after oxidative stress (7-11). The signal for recognition and degradation of oxidized proteins by the 20 S proteasome is unknown but has been suggested to involve (i) exposure of hydrophobic surfaces after oxidative modification, (ii) recognition of molecular "markers" associated with the oxidative modification of specific amino acid side chains, and (iii) increases in the conformational flexibility of oxidized proteins (12-16).To distinguish which signals are involved in targeting an oxidized protein for degradation by the 20 S proteasome, we have investigated the mech...

We have used electrospray ionization mass spectrometry (ESI-MS), circular dichroism (CD), and fluorescence spectroscopy to investigate the secondary and tertiary structural consequences that result from oxidative modification of methionine residues in wheat germ calmodulin (CaM), and prevent activation of the plasma membrane Ca-ATPase. Using ESI-MS, we have measured rates of modification and molecular mass distributions of oxidatively modified CaM species (CaMox) resulting from exposure to H2O2. From these rates, we find that oxidative modification of methionine to the corresponding methionine sulfoxide does not predispose CaM to further oxidative modification. These results indicate that methionine oxidation results in no large-scale alterations in the tertiary structure of CaMox, because the rates of oxidative modification of individual methionines are directly related to their solvent exposure. Likewise, CD measurements indicate that methionine oxidation results in little change in the apparent alpha-helical content at 28 degrees C, and only a small (0.3 +/- 0.1 kcal mol(-1)) decrease in thermal stability, suggesting the disruption of a limited number of specific noncovalent interactions. Fluorescence lifetime, anisotropy, and quenching measurements of N-(1-pyrenyl)-maleimide (PMal) covalently bound to Cys26 indicate local structural changes around PMal in the amino-terminal domain in response to oxidative modification of methionine residues in the carboxyl-terminal domain. Because the opposing globular domains remain spatially distant in both native and oxidatively modified CaM, the oxidative modification of methionines in the carboxyl-terminal domain are suggested to modify the conformation of the amino-terminal domain through alterations in the structural features involving the interdomain central helix. The structural basis for the linkage between oxidative modification and these global conformational changes is discussed in terms of possible alterations in specific noncovalent interactions that have previously been suggested to stabilize the central helix in CaM.

We have investigated the ability of methionine sulfoxide reductase (MsrA) to maintain optimal calmodulin (CaM) function through the repair of oxidized methionines, which have been shown to accumulate within CaM in senescent brain [Gao, J., Yin, D. H., Yao, Y., Williams, T. D., and Squier, T. C. (1998) Biochemistry 37, 9536-9548]. Oxidatively modified calmodulin (CaMox) isolated from senescent brain or obtained by in vitro oxidation was incubated with MsrA. This treatment restores the functional ability of CaMox to activate the plasma membrane (PM) Ca-ATPase, confirming that (i) the decreased ability of CaM isolated from senescent animals to activate the PM Ca-ATPase results solely from methionine sulfoxide formation and (ii) MsrA can repair methionine sulfoxides within cytosolic proteins. We have used electrospray ionization mass spectrometry to investigate the extent and rates of methionine sulfoxide repair within CaMox. Upon exhaustive repair by MsrA, there remains a distribution of methionine sulfoxides within functionally reactivated CaMox, which varies from three to eight methionine sulfoxides. The rates of repair of methionine sulfoxides within individual tryptic fragments of CaMox vary by a factor of 2, where methionine sulfoxides located within hydrophobic sequences are repaired in preference to methionines that are more solvent accessible within the native structure. However, no single methionine sulfoxide is completely repaired in all CaM oxiforms. Decreases in the alpha-helical content and a disruption of the tertiary structure of CaM have previously been shown to result from methionine oxidation. Repair of selected methionine sulfoxides in CaMox by MsrA results in a partial refolding of the secondary structure, suggesting that MsrA repairs methionine sulfoxides within unfolded sequences until native-like structure and function are re-attained. The ability of CaMox isolated from senescent brain to fully activate the PM Ca-ATPase following repair by MsrA suggests the specific activity of MsrA is insufficient to maintain CaM function in aging brain. These results are discussed in terms of the possible regulatory role MsrA may play in the modulation of CaM function and calcium homeostasis under conditions of oxidative stress.

We have used fluorescence spectroscopy to investigate the average structure and extent of conformational heterogeneity associated with the central helix in calmodulin (CaM), a sequence that contributes to calcium binding sites 2 and 3 and connects the amino- and carboxyl-terminal globular domains. Using site-directed mutagenesis, a double mutant was constructed involving conservative substitution of Tyr(99) --> Trp(99) and Leu(69) --> Cys(69) with no significant effect on the secondary structure of CaM. These mutation sites are at opposite ends of the central helix. Trp(99) acts as a fluorescence resonance energy transfer (FRET) donor in distance measurements of the conformation of the central helix. Cys(69) provides a reactive group for the covalent attachment of the FRET acceptor 5-((((2-iodoacetyl)amino)ethyl)amino)naphthalene-1-sulfonic acid (IAEDANS). AEDANS-modified CaM fully activates the plasma membrane (PM) Ca-ATPase, indicating that the native structure is retained following site-directed mutagenesis and chemical modification. We find that the average spatial separation between Trp(99) and AEDANS covalently bound to Cys(69) decreases by approximately 7 +/- 2 A upon calcium binding. However, irrespective of calcium binding, there is little change in the conformational heterogeneity associated with the central helix under physiologically relevant conditions (i.e., pH 7.5, 0.1 M KCl). These results indicate that calcium activation alters the spatial arrangement of the opposing globular domains between two defined conformations. In contrast, under conditions of low ionic strength or pH the structure of CaM is altered and the conformational heterogeneity of the central helix is decreased upon calcium activation. These results suggest the presence of important ionizable groups that affect the structure of the central helix, which may play an important role in mediating the ability of CaM to rapidly bind and activate target proteins.

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