The LIM/double zinc-finger motif functions as a protein dimerization domain

The cysteine-rich protein family of highly related LIM domain proteins

Here we describe a family of closely related LIM domain proteins in avian cells. The LIM motif defines a zinc-binding domain that is found in a variety of transcriptional regulators, proto-oncogene products, and proteins associated with sites of cell-substratum contact. One type of LIM-domain protein, called the cysteine-rich protein (CRP), is characterized by the presence of two LIM domains linked to short glycine-rich repeats and a potential nuclear localization signal. We have identified and characterized two evolutionarily conserved members of the CRP family, CRP1 and CRP2, in chicken and quail. Expression of the genes encoding both CRP1 and CRP2 is differentially regulated in normal versus transformed cells, raising the possibility that members of the CRP family may function in control of cell growth and differentiation.

Growth factor-induced tyrosine phosphorylation of Hrs, a novel 115-kilodalton protein with a structurally conserved putative zinc finger domain

The activation of growth factor receptor tyrosine kinases leads to tyrosine phosphorylation of many intracellular proteins which are thought to play crucial roles in growth factor signaling pathways. We previously showed that tyrosine phosphorylation of a 115-kDa protein is rapidly induced in cells treated with hepatocyte growth factor. To clarify the structure and possible function of the 115-kDa protein (designated Hrs for hepatocyte growth factor-regulated tyrosine kinase substrate), we purified this protein from B16-F1 mouse melanoma cells by anti-phosphotyrosine immunoaffinity chromatography and determined its partial amino acid sequences. On the basis of the amino acid sequences, we molecularly cloned the cDNA for mouse Hrs. The nucleotide sequence of the cDNA revealed that Hrs is a novel 775-amino-acid protein with a putative zinc finger domain that is structurally conserved in several other proteins. This protein also contained a proline-rich region and a proline- and glutamine-rich region. The expression of Hrs mRNA was detected in all adult mouse tissues tested and also in embryos. To analyze the Hrs cDNA product, we prepared a polyclonal antibody against bacterially expressed Hrs. Using this antibody, we showed by subcellular fractionation that Hrs is localized to the cytoplasm; we also showed that that tyrosine phosphorylation of Hrs is induced in cells treated with epidermal growth factor or platelet-derived growth factor. These results suggest that Hrs plays a unique and important role in the signaling pathway of growth factors.

The LIM domain homeobox gene isl-1 is a positive regulator of islet cell-specific proglucagon gene transcription

The LIM domain homeobox gene islet 1 (isl-1) is expressed in the embryonic nervous system and may be an early marker of motor neuron specification. isl-1 is expressed in all 4 islet cell types, but a role for isl-1 in the regulation of insulin gene expression has not been demonstrated, and the genetic targets for isl-1 in the pancreas remain unknown. We show here that the proximal rat proglucagon gene promoter binds an amino-terminally truncated Trp-E-isl-1 fusion protein that lacks the LIM domains. The proglucagon gene promoter also binds full-length in vitro translated isl-1 containing the intact LIM domains. isl-1 antisera detects binding of proglucagon gene sequences to isl-1 present in a slowly-migrating complex in nuclear extracts from InR1-G9 islet cells. The transcriptional properties of the proglucagon gene promoter sequences that bind isl-1 (designated Ga, Gb, and Gc) were assessed after transfection of reporter genes into wild type and isl-1-antisense (isl-1(AS)) InR1-G9 islet cells. The proximal proglucagon gene (Ga) promoter sequence reduced TK-CAT activity by approximately 50%, but no change in the activity of the Ga-TK-CAT plasmid was seen after transfection of isl-1(AS) InR1-G9 cells. In contrast, the Gb/Gc sites activated transcription 2-3 fold in wild type InR1-G9 cells, and the isl-1-dependent activation of gene transcription through the Gb/Gc element was eliminated following transfection of isl-1(AS) InR1-G9 cells. These data demonstrate that the LIM domain homeobox gene isl-1 1) is not constrained from DNA binding by its LIM domains and 2) functions as a positive regulator of proglucagon gene transcription in the endocrine pancreas.

P-Lim, a LIM homeodomain factor, is expressed during pituitary organ and cell commitment and synergizes with Pit-1

A pituitary LIM homeodomain factor, P-Lim, is expressed as Rathke's pouch forms and as specific pituitary cell phenotypes are established, suggesting functional roles throughout pituitary development. While selectively expressed in both anterior and intermediate pituitary in mature mice, P-Lim is also transiently expressed in the developing ventral neural cord and brainstem. P-Lim binds to and activates the promoter of the alpha-glycoprotein subunit gene, a marker of early pituitary development, and synergizes with Pit-1 in transcriptional activation of genes encoding terminal differentiation markers. The LIM domain of P-Lim specifically interacts with the Pit-1 POU domain and is required for synergistic interactions with Pit-1, but not for basal transcriptional activation events.