Investigating the functional significance of evolutionarily conserved protein motifs of the Drosophila melanogaster HOX protein, Sex combs reduced

Bilaterans share a common anterior-posterior (A-P) axis that is patterned by the Homeotic selector (Hox) genes. In Drosophila melanogaster, Hox gene expression in spatially restricted domains along the A-P axis of the embryo determines segmental identity. Identifying the genetic mechanisms of HOX control of development is essential for understanding body patterning in animals. I identified and characterized the role of evolutionarily conserved protein domains of the HOX protein, Sex combs reduced (SCR), in protein function. SCR is required for establishing the identity of both the labial and prothoracic segments. To identify regions of functional importance, 15 Scr point mutant alleles were sequenced and grouped into three allelic classes: null, hypomorphic and hypomorphic-antimorph. Null alleles were nonsense mutations resulting in truncation and loss of highly conserved protein domains. Hypomorphic alleles were missense and small deletion mutations in highly conserved protein domains, including the DYTQL motif, YPWM motif and C-terminal domain (CTD). Examination of the affect of changes in conserved domains on three SCR dependent phenotypes revealed multiple examples of differential pleiotropy: the observation that HOX proteins are made up of small independently acting peptide motifs that alone make small contributions to activity. The third class is the hypomorphic-antimorphic allele, Scr14, which is a missense mutation in the octapeptide motif. The mechanism of Scr14 antimorphy may be the acquisition of a leucine zipper motif in the octapeptide and LASCY motifs. This leucine zipper motif confers oligomerization potential in vitro, and allows inhibition of Scr activity by SCR14 in vivo in a reciprocal manner. Lastly, I tested the genetic model that SCR and the HOX protein, Proboscipedia (PB), form a complex to determine proboscis identity in the labial segment. Co-immunoprecipitation assays were unsuccessful at detecting a biochemical interaction between PB and SCR, indicating that the mechanism for proboscis determination may not involve complex formation between these two proteins. Together, these results demonstrate that the contribution of evolutionarily conserved HOX protein domains to HOX control of development is complex