Ikaros family members from the agnathan Myxine glutinosa and the urochordate Oikopleura dioica: emergence of an essential transcription factor for adaptive immunity.

The Ikaros multigene family encodes a number of zinc finger transcription factors that play key roles in vertebrate hemopoietic stem cell differentiation and the generation of B, T, and NK cell lineages. In this study, we describe the identification and characterization of an Ikaros family-like (IFL) protein from the agnathan hagfish Myxine glutinosa and the marine urochordate Oikopleura dioica, both of which lie on the evolutionary boundary between the vertebrates and invertebrates. The IFL molecules identified in these animals displayed high conservation in the zinc finger motifs critical for DNA binding and dimerization in comparison with those of jawed vertebrates. Expression of the IFL gene in hagfish was strongest in blood, intestine, and gills. In O. dioica, transcription from the IFL gene was initiated at or around the time of hatching and maintained throughout the life span of the animal. In situ hybridization localized O. dioica IFL expression to the Fol cells, which are responsible for generating the food filter of the house. Biochemical analysis of the DNA binding and dimerization domains from M. glutinosa and O. dioici IFLs showed that M. glutinosa behaves as a true Ikaros family member. Taken together, these results indicate that the properties associated with the Ikaros family preceded the emergence of the jawed vertebrates and thus adaptive immunity.

Selective dimerization of a C2H2 zinc finger subfamily.

The C2H2 zinc finger is the most prevalent protein motif in the mammalian proteome. Two C2H2 fingers in Ikaros are dedicated to homotypic interactions between family members. We show here that these fingers comprise a bona fide dimerization domain. Dimerization is highly selective, however, as homologous domains from the TRPS-1 and Drosophila Hunchback proteins support homodimerization, but not heterodimerization with Ikaros. Ikaros-Hunchback selectivity is determined by 11 residues concentrated within the alpha-helical regions typically involved in base recognition. Preferential homodimerization of one chimeric protein predicts a parallel dimer interface and establishes the feasibility of creating novel dimer specificities. These results demonstrate that the C2H2 motif provides a versatile platform for both sequence-specific protein-nucleic acid interactions and highly specific dimerization.