The PAX3 and 7 homeodomains have evolved unique determinants that influence DNA-binding, structure and communication with the paired domain

The PAX (paired box) family is a collection of metazoan transcription factors defined by the paired domain, which confers sequence-specific DNA-binding. Ancestral PAX proteins also contained a homeodomain, which can communicate with the paired domain to modulate DNA-binding. In the present study, we sought to identify determinants of this functional interaction using the paralogous PAX3 and 7 proteins. First, we evaluated a group of heterologous paired domains and homeodomains for the ability to bind DNA cooperatively through formation of a ternary complex (paired domain:homeodomain:DNA). This revealed that capacity for ternary complex formation was unique to the PAX3 and 7 homeodomains and therefore not simply a consequence of DNA-binding. We also found PAX3 and 7 were distinguished by an extended region of conservation N-terminal to the homeodomain (NTE). Phylogenetic analyses established the NTE was restricted to PAX3/7 orthologs of segmented metazoans, indicating it arose in a bilaterian precursor prior to separation of deuterostomes and protostomes. In DNA-binding assays, presence of the NTE caused a decrease in monomeric binding by the PAX3 homeodomain that reflected a lack of secondary structure in 1D-1H-NMR. Nevertheless, this inhibitory effect could be overcome by homeodomain dimerization or cooperative binding with the paired domain, establishing that protein interactions could induce homeodomain folding in the presence of the NTE. Strikingly, the PAX7 counterpart did not impair homeodomain binding, revealing inherent differences that could account for its distinct target profile in vivo. Collectively, these findings identify critical determinants of PAX3 and 7 activity, which contribute to their functional diversification.