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A COMPARISON OF DIFFERENT PREPARATIONS IF BOVINE BRAIN CALMODULIN

During our studies of the conformational changes of calmodulin in solution by 'H-NMR we have noticed differences in the spectra of the calcium-bound form of the protein in different preparations. We have compared samples both prepared by us and sent to us from several other laboratories. Although most of the samples, called 'normal', give spectra similar to those published (K. Seamon. 1980. Biochemistry 19: 207.). several are significantly different. The most striking differences appear in the aromatic region of the spectra, suggesting a change in the positions of the aromatic side chains with respect to one another. A change of this kind could also arise from small amounts of contaminating material absorbing in the same region of the spectrum or from binding of an impurity to the protein. In order to determine the nature of the differences we have observed, we have used techniques other than NMR to compare representative samples: Type A are those which give 'normal' NMR spectra and Type B are those which give 'changed' spectra. We have noted a difference in the UV spectra of Type B samples. The relative intensities of the absorbance maxima have shifted so that 276 nm is no longer the highest point in the spectrum. Again, this observation could arise from contamination which absorbs in the ultra-violet. The addition of trace amounts of 2mercaptoethanol. EDTA, or sodium azide, any of which could conceivably be present, to a sample of Type A changes the spectrum but does not generate a Type B spectrum. A difference spectrum of Types A and B gives a maximum at about 260 nm. Interestingly, the peak that elutes directly after calmodulin off DEAE-Sephadex during purification has A, = 260 nm. The identity of this material is unknown to us. It does not appear as a protein band on 7.5% polyacrylamide gels. The two types of calmodulin also have different mobilities when electrophoresed on 12.5% polyacrylamide gels at pH 8.6. In the absence of added Car' or EGTA, Type B has a higher mohility than Type A, and its mobility is unchanged by the addition of either EGTA or Ca+' to the sample prior to electrophoresis. The addition of trace amounts of EGTA, sodium azide, 2-mercaptoethanol, or nucleotide to Type A does not generate a sample with Type B mobility. The isoelectric focusing patterns of Types A and B are quite different. In our hands, even Type A gives multiple bands when 150 pg are focused. We observe 5 bands, with the central band being the major species. Type B also gives 5 bands, but the relative intensities are very different from Type A: there are two major bands of equal intensity that focus on either side of the major Type A band that now appears as a minor band, and two more highly acidic bands. The electrophoresis and electrofocusing data suggest that the differences observed between Types A and B are due to changes in the protein, rather than