Abstract
In 1932, Müller first used the term "antimorphic" to describe mutant alleles that have an effect that is antagonistic to that of the wild-type allele from which they were derived. In a previous characterization of mutant alleles of the Drosophila melanogaster Hox gene, Sex combs reduced (Scr), we identified the missense, antimorphic allele Scr(14), which is a Ser10-to-Leu change in the N-terminally located, bilateran-specific octapeptide motif. Here we propose that the cause of Scr(14) antimorphy is the acquisition of a leucine zipper oligomerization motif spanning the octapeptide motif and adjacently located protostome-specific LASCY motif. Analysis of the primary and predicted secondary structures of the SCR N-terminus suggests that while the SCR(+) encodes a short, α-helical region containing one putative heptad repeat, the same region in SCR(14) encodes a longer, α-helical region containing two putative heptad repeats. In addition, in vitro cross-linking assays demonstrated strong oligomerization of SCR(14) but not SCR(+). For in vivo sex comb formation, we observed reciprocal inhibition of endogenous SCR(+) and SCR(14) activity by ectopic expression of truncated SCR(14) and SCR(+) peptides, respectively. The acquisition of an oligomerization domain in SCR(14) presents a novel mechanism of antimorphy relative to the dominant negative mechanism, which maintains oligomerization between the wild-type and mutant protein subunits.