Study of the charge-state model for electrophoretic variation using isoelectric focusing of esterase-5 from Drosophila pseudoobscura

The separation of whale myoglobins with two-dimensional electrophoresis

Five myoglobins (sperm whale, Sei whale, Hubbs' beaked whale, pilot whale, and Amazon River dolphin) were examined using two-dimensional electrophoresis. Previous reports indicated that none of these proteins could be separated by using denaturing (in the presence of 8-9 M urea) isoelectric focusing. This result is confirmed in the present study. However, all the proteins could be separated by using denaturing nonequilibrium pH-gradient electrophoresis in the first dimension. Additionally, all the myoglobins have characteristic mobilities in the second dimension (sodium dodecyl sulfate), but these mobilities do not correspond to the molecular weights of the proteins. We conclude that two-dimensional electrophoresis can be more sensitive to differences in primary protein structure than previous studies indicate and that the assessment seems to be incorrect that this technique can separate only proteins that have a unit charge difference.

Macromolecular interaction and the electrophoretic mobility of esterase-5 from Drosophila pseudoobscura

Esterase-5 is one of the most polymorphic loci in Drosophila pseudoobscura. Some variants reportedly produce a dimeric enzyme, while a few produce a monomeric form. This paper reports the finding that during electrophoresis ESTERASE-5 exists in a dynamic equilibrium between monomers and dimers, an equilibrium that is dependent on the running temperature of the gels. This is shown by a series of analytical electrophoresis experiments in which the apparent molecular weights of several variants are determined at four different temperatures. Increasing temperatures result in a linear decrease in the logarithm of apparent molecular weights. Macromolecular interactions thus are a significant determinant of EST-5 electrophoretic mobility.

The sensitivity of isoelectric focusing and electrophoresis in the detection of sequence differences in proteins

Fourteen myoglobins of known sequence were examined by isoelectric focusing with and without urea. The 14 sequences formed six distinct mobility classes on gels without urea and three classes on those with urea. For these proteins, isoelectric focusing provides no advantage over single, nonequilibrium, nondenaturing gels in the total number of distinguishable mobility classes. Only major charge differences, resulting from the changes in the total numbers of acidic and basic amino acids, can be detected on gels with urea, indicating that denaturation by urea alters proteins so that small differences in ionization are eliminated.

Genetic divergence between rodent species assessed by using two-dimensional electrophoresis

O'Farrell's technique of two-dimensional gel electrophoresis (2-DGE) has previously been applied to the study of intrapopulation genetic variation. This approach assays a larger, and in part nonoverlapping, cohort of protein encoding loci compared to conventional one-dimensional electrophoretic procedures (SGE) and has revealed substantially lower levels of mean heterozygosity. Here we extend this approach to analyze levels and patterns of genetic differentiation between species. We have used 2-DGE to compare an average of 189 polypeptides between six species of wild mice representing levels of evolutionary divergence ranging from different subspecies to different families. The magnitude of protein divergence estimated by 2-DGE was on the average only about one-half that predicted by SGE. This discrepancy may result from differences in sensitivities between the techniques or differences in the mean level of variation and divergence between the sets of loci assayed by the two methods. Nonetheless, the ranking of genetic distances by 2-DGE was identical to that by SGE. Thus, these results support the use of the simpler SGE techniques to estimate relative levels of genetic divergence.

Genetic heterozygosity in a natural population of Mus musculus assessed using two-dimensional electrophoresis

Since the introduction of gel electrophoresis to population genetics, estimates of genic heterozygosity at allozyme loci have been made for many organisms. However, attempts to extend the range of proteins surveyed have been hampered by concern that there may have been bias in the loci sampled: nearly all proteins for which population data are available are soluble in low-salt extracts and most are enzymes belonging to particular groups such as the nonspecific esterases and phosphatases, various dehydrogenases and the enzymes of the glycolytic pathways and the citric acid cycle. Until recently, there were no techniques for estimating protein variation which were without such bias. Two-dimensional gel electrophoresis, however, offered hope of overcoming this bias, and it has proved useful for the resolution of proteins in crude homogenates and for estimating genetic variation. We have examined heterozygosity in a wild population of the house mouse (Mus musculus) by means of two-dimensional electrophoresis of whole kidney. As we report here, the observed level (2%) is substantially below the level detected by starch gel electrophoresis. Our results corroborate similar differences observed in Drosophila and man.

Chemical basis of the electrophoretic variation observed at the alcohol dehydrogenase locus of Drosophila melanogaster

The amino acid substitution responsible for the different electrophoretic mobility of the ADHs alleloenzyme and the ADHf alleloenzyme of the alcohol dehydrogenase from a Nigerian population of Drosophila melanogaster has been established as lysine (ADHs) for threonine (ADHf). This result is discussed with reference to the charge state model of electrophoretic variation, in conjunction with other know substitutions at this locus. It is concluded that electrophoretic methods should be capable of distinguishing many alleloenzymes which have identical isoelectric points without recourse to explanations involving conformational variability.

Charge-state and charge-continuum models in electrophoresis and isoelectric focusing of genetic variants

A model which combines the charge-state and charge-continuum models is considered in terms of the pH at which the electrophoretic analysis is made. "Unit" charge changes are obtained when the mutated charged group is at least 2 pH units apart from the pI of the protein. "Fractional" charge changes are found when the differences between the pK of the mutated charged group and the pI of the protein becomes progressively smaller, or when the pK of the ionizable group lies on the "wrong" side of the protein pI. It is possible to measure the pK of a modified group in a protein when the delta pI per protonic unit in a given family of proteins (e.g., haemoglobin mutants) is less than unity.