HinZip: Combining Hin Recombinase and FosW to Mimic HD-Zip Plant Proteins

Small customized proteins that bind specific DNA sequences in a genome could serve as powerful tools for synthetic biology and therapeutic applications. These proteins could regulate gene circuits or act as precision-targeted inhibitors in disease networks. Here, we designed HinZip, a protein engineered to bind a unique 24+ base-pair DNA sequence with high affinity and specificity, thereby minimizing off-target effects. HinZip is inspired by the HD-Zip (homeodomain-leucine zipper) transcription factor family, which exists only in plants. No high-resolution structures exist for HD-Zip: genome-wide analyses indicate that HD-Zips use a homeodomain to bind DNA and a leucine zipper for dimerization. To emulate this functionality, we fused the Hin recombinase DNA-binding domain with the FosW leucine zipper. Electrophoretic mobility shift assays confirmed HinZip's cooperative binding to a 29 base-pair inverted HixC palindrome (K_d = 17 nM), with no detectable binding to nonspecific DNA at protein concentrations up to 2 μM. Circular dichroism and dynamic light scattering further support dimer formation. Additionally, the bacterial one-hybrid assay demonstrated HinZip's sequence-specific binding in cellulo. Even in the absence of structural guidance, we successfully designed a functional "frankenprotein" by integrating unrelated protein modules. This work underscores the feasibility of engineering bespoke DNA-binding proteins for targeted genomic interactions.