A cytoplasmic 57-kDa protein that is required for translation of picornavirus RNA by internal ribosomal entry is identical to the nuclear pyrimidine tract-binding protein (
ABSTRACTInitiation of translation of the RNA genomes of picornaviruses such as poliovirus and encephalomyocarditis vius is cap-independent and results from interaction of ribosomes with a segment of the 5' noncoding region of these mRNAs termed the internal ribosomal entry site. Genetic and biochemical studies have previously shown that a 57-kDa cytoplasmic RNA-binding protein (p57) plays an essential rol in this translation mechanism. We have now found that p57 shares physical, biochemical, and antigenic properties with the pyrimidine tract-binding protein (PTB), a nuclear protein that
Truncation mutations of the GLI3 zinc finger transcription factor can cause Greig cephalopolysyndactyly syndrome (GCPS), Pallister-Hall syndrome (PHS), and postaxial polydactyly type A (PAP-A). GLI3 is homologous to Drosophila Cubitus interruptus (Ci), which regulates the patched (ptc), gooseberry (gsb), and decapentaplegic (dpp) genes. Ci is sequestered in the cytoplasm and is subject to posttranslational processing whereby the full-length transcriptional activator form (Ci 155 ) can be cleaved to a repressor form (Ci 75 ). Under hedgehog signaling, the Ci 155 form translocates to the nucleus whereas in the absence of hedgehog, the Ci 75 form translocates to the nucleus. Based on the correlation of GLI3 truncation mutations and the human phenotypes, we hypothesized that GLI3 shows transcriptional activation or repression activity and subcellular localization similar to Ci. Here we show that full-length GLI3 localizes to the cytoplasm and activates PTCH1 expression, which is similar to fulllength Ci 155 . PHS mutant protein (GLI3-PHS) localizes to the nucleus and represses GLI3-activated PTCH1 expression, which is similar to Ci 75 . The GCPS mutant protein has no effect on GLI3-activated PTCH1 transcription, consistent with the role of haploinsufficiency in this disorder. The PAP-A mutant protein (GLI3-PAP-A) showed less specific subcellular localization but still inhibited GLI3-activated PTCH1 transcription, suggesting it may be a weaker allele than the GLI3-PHS mutation. These data show that GLI3 mutations in humans mimic functional effects of the Drosophila ci gene and correlate with the distinct effects of these mutations on human development.The GLI3 gene originally was cloned by homology to GLI and subsequently was shown to be interrupted by translocations in patients with the Greig cephalopolysyndactyly syndrome (GCPS) (1). Subsequently, it was demonstrated that mutations in GLI3 can cause the Pallister-Hall syndrome (PHS) (2) and postaxial polydactyly type A (PAP-A) (3). These findings raised the question of how distinct human developmental disorders (GCPS and PHS) and isolated anomalies (PAP-A) can be caused by mutations in a single gene. The GCPS mutations that have been described (truncations, deletions, point mutations, and translocations) support the previous assertions that haploinsufficiency of GLI3 causes GCPS whereas frameshift mutations alone are known to cause PHS and PAP-A. Because all three disorders can be caused by frameshift mutations, we chose to investigate the potential correlation of the position of truncation mutations and the effect of those mutations on GLI3 localization and function as a possible explanation for the distinct phenotypes (4). The model is based on the homology of GLI3 to the Drosophila cubitus interruptus gene product, Ci (5). In Drosophila, Ci exists as a 155-kDa full-length form (Ci 155
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