Evolutionary relationship between the cysteine and histidine rich domains (CHORDs) and Btk-type zinc fingers

Cysteine and histidine rich domains (CHORDs), implicated in immunity and disease resistance signaling in plants, and in development and signal transduction in muscles and tumorigenesis in animals, are seen to have a cylindrical three-dimensional structure stabilized by the tetrahedral chelation of two zinc ions. CHORDs are regarded as novel zinc-binding domains and classified independently in Pfam and ECOD. Our sequence and structure analysis reveals that both the zinc-binding sites in CHORD possess a zinc ribbon fold and are likely related to each other by duplication and circular permutation. Interestingly, we also detect an evolutionary relationship between each of the CHORD zinc fingers (ZFs) and the Bruton's tyrosine kinase (Btk)-type ZF of the zinc ribbon fold group. Btk_ZF is found in eukaryotic Tec kinase family proteins that are also implicated in signaling pathways in several lineages of hematopoietic cells involved in mammalian immunity. Our analysis suggests that the unique zinc-stabilized fold seen only in the CHORD and Btk_ZFs likely emerged specifically in eukaryotes to mediate diverse signaling pathways.