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

Abstract

VARIOUS neutral models have been proposed to account for the observed levels of enzyme polymorphism in natural populations^1–5. One such model, the charge-state or ladder-rung model², has been suggested to be appropriate for the available electrophoretic data. The model assumes that the only amino acid substitutions detectable by electrophoresis would involve unit charge changes—substitution of a charged for an uncharged residue—and because of charge redundancy alleles would fall into phenotypic classes of equal net charge^3,4. Thus an electrophoretic variant would be a heterogeneous molecular entity, rather than the product of a unique allele. It was proposed that these charge classes would be evenly spaced in electrophoretic mobility, because unit charge changes would be reflected as unit jumps in mobility, but Johnson⁶ has questioned this on the grounds that no two charged group substitutions should result in identical net charge changes. He argued that there would be no charge redundancy and that there would be a continuum of charged variants. It was also recognised that while many substitutions involving uncharged residues would probably go undetected, some could cause conformational changes which would be resolved by electrophoresis. This concern has been borne out by the electrophoretic data: although many enzymes fit the expectations for neutral alleles contingent on the charge-state model^7,8, others, including some esterases, do not. The mobilities of variants of these esterases are not evenly spaced, and rare variants are often observed at intermediate as well as extreme electrophoretic mobilities. These characteristics have been attributed to the presence of conformation, as well as charge, variability. We have investigated the relative importance of charge and conformational change in producing electrophoretic variation by examining variants at the Est-5 locus of Drosophila pseudoobscura by isoelectric focusing; this allows charge balance to be examined in the absence of conformational variation. The results indicate that while the observed charge changes fit a charge-state model, conformational differences exist which may account for a substantial amount of the additional variation revealed at this locus by conventional electrophoresis.