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

Evolutionary genetics of Drosophila esterases

Over 30 carboxylester hydrolases have been identified in D. melanogaster. Most are classified as acetyl, carboxyl or cholinesterases. Sequence similarities among most of the carboxyl and all the cholinesterases so far characterised from D. melanogaster and other eukaryotes justify recognition of a carboxyl/cholinesterase multigene family. This family shows minimal sequence similarities with other esterases but crystallographic data for a few non-drosophilid enzymes show that the family shares a distinctive overall structure with some other carboxyl and aryl esterases, so they are all put in one superfamily of/beta hydrolases. Fifteen esterase genes have been mapped in D. melanogaster and twelve are clustered at two chromosomal sites. The constitution of each cluster varies across Drosophila species but two carboxyl esterases in one cluster are sufficiently conserved that their homologues can be identified among enzymes conferring insecticide resistance in other Diptera. Sequence differences between two other esterases, the EST6 carboxyl esterase and acetylcholinesterase, have been interpreted against the consensus super-secondary structure for the carboxyl/cholinesterase multigene family; their sequence differences are widely dispersed across the structure and include substantial divergence in substrate binding sites and the active site gorge. This also applies when EST6 is compared across species where differences in its expression indicate a difference in function. However, comparisons within and among species where EST6 expression is conserved show that many aspects of the predicted super-secondary structure are tightly conserved. Two notable exceptions are a pair of polymorphisms in the substrate binding site of the enzyme in D. melanogaster. These polymorphisms are associated with differences in substrate interactions in vitro and demographic data indicate that the alternative forms are not selectively equivalent in vivo.

Perturbation-reperturbation test of selection vs. hitchhiking of the two major alleles of Esterase-5 in Drosophila pseudoobscura

A perturbation-reperturbation tests selective neutrality of 100/100/100/100/100 and 106/100/100/100/100, the two most common alleles at the highly polymorphic X-linked locus Esterase-5 in Drosophila pseudoobscura. A total of 22 replicate populations are set up in cages, 11 start at a high frequency of 76% (U) and 11 at a low frequency of 21% (N) of the 106 allele. Allele frequencies change directionally and decrease in both U and N populations as groups and reach equilibria of 60 and 14%, respectively, after 200-300 days. These changes suggest natural selection. A hypothesis of balancing selection accounts for the pattern and predicts a dynamic equilibrium. A rival neutral hypothesis accounts for the pattern equally well by postulating hitchhiking and breakup of linkage leaving the Est-5 variants to drift at neutral equilibria. A reperturbation of allele frequencies in each population, creating 22 additional reperturbed populations EN and EU, with the original populations as controls, directly addresses the question of balancing selection or hitchhiking and breakup of linkage effects. Allele frequencies do not change directionally among the reperturbed populations as a group. The hypothesis of balancing selection is rejected in favor of the hypothesis of initial hitchhiking and dissipated linkage effects. The power of the experimental design to detect selection is studied by simulation. Within the limits of power set by the design, it is concluded that the 100 and 106 are iso-fitness alleles of Est-5 under the environmental conditions of the laboratory populations. The requirements of a method of perturbation and reperturbation are discussed.

Theory of electrophoresis of hybridizing enzymes with kinetic control: implications for population genetics of electrophoretic markers

A phenomenological theory of zone electrophoresis has been formulated for reversible, kinetically controlled self-dimerization and hybridization of proteins. The computed electrophoretic patterns simulate many of the observed features of certain Drosophila enzymes. The patterns for rapid self-dimerization show a unimodal reaction zone over a wide range of rates of dissociation of the dimer, the migration velocity increasing with increasing rate of dissociation. Those for slow rates of both dimerization and dissociation are bimodal reaction zones exhibiting a peak composed largely of dimer and one largely of monomer separated by a low plateau. The nature of the pattern for hybridization of two dimerizing variants of a polymorphic protein is strikingly dependent upon the rates of reaction. For rapid association and very slow dissociation the pattern exhibits three zones corresponding to the two homodimers and the heterodimer. Very rapid rates of dissociation give two monomer zones. Intermediate rates of dissociation also give two zones, but here they are uncoupled reaction zones; each corresponds to an interconverting mixture of one of the homodimers and its monomer, the heterodimer having dissociated completely during early stages of electrophoresis. The transition from a three-zone pattern to this two-zone pattern has been examined in detail, and a mechanism is given. Practical implications are considered.