Identification and characterization of Grainyhead-like Epithelial transactivator (GET-1), a novel mammalian Grainyhead-like factor
LMO-4 is an LIM-only factor that is highly expressed in many epithelial cells, including those of the epidermis and hair follicles. Because LMOs may function by interacting with DNA-binding proteins, we have used the yeast two-hybrid system to screen mouse skin libraries for LMO-4-interacting DNA-binding proteins. In this screen, we isolated a novel LMO-4-interacting factor highly related to the Drosophila gene Grainyhead. Grainyhead is epidermally expressed and carries out important functions in cuticular formation in the fly embryo. With the identification of this novel mammalian Grainyhead-like gene, referred to as Grainyhead-like epithelial transactivator 1 (GET-1), the known members of the mammalian Grainyhead-like gene family are extended to six, falling into two classes based on sequence homology. Of interest, the expression pattern of GET-1 is similar to that of Drosophila Grainyhead with highest expression in the somatic ectoderm/epidermis, but GET-1 is additionally expressed in epithelial cells of gastrointestinal, genitourinary, and respiratory tracks. The GET-1 protein localizes to the nucleus and binds to at least one Grainyhead DNA-binding site. The GET-1 DNA-binding domain maps to a region containing homology to the Drosophila Grainyhead DNA-binding domain. GET-1 homodimerizes in solution by means of a short C-terminally located domain that is homologous to other Grainyhead-like genes. A short domain located between amino acids 100 and 190, which bears no homology to known transactivation domains, is sufficient to confer transactivation to the heterologous GAL4 DNA-binding domain. In addition, GET-1 appears to contain repression domains consistent with the observation that Grainyhead and other mammalian Grainyhead-like genes can act both as activators and repressors. These data suggest that GET-1 is a transcriptional regulator that may perform important functions in epithelial tissues of mammals.