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1.  Characterization of the internal IRES element of the zebrafish connexin55.5 reveals functional implication of the polypyrimidine tract binding protein 
Background
Connexin55.5 (Cx55.5) is a gap junction protein with horizontal cell-restricted expression in zebrafish accumulating at dendritic sites within the receptor-horizontal cell complex in form of hemichannels where light-dependent plasticity occurs. This connexin is the first example of a gap junction protein processed to form two protein isoforms from a monocistronic message by an IRES mediated process. The nuclear occurrence of a carboxy-terminal fragment of this protein provides evidence that this gap junction protein may participate in a putative cytoplasmic to nuclear signal transfer.
Results
We characterized the IRES element of Cx55.5 in terms of sequence elements necessary for its activity and protein factor(s), which may play a role for its function. Two stretches of polypyrimidine tracts designated PPT1 and PPT2 which influence the IRES activity of this neuronal gap junction protein were identified. Selective deletion of PPT1 results in an appreciable decrease of the IRES activity, while the deletion of PPT2 results in a complete loss. RNA-EMSA and UV-cross linking experiments showed that protein complexes bind to this IRES element, of which the polypyrimidine tract binding protein (PTB) was identified as one of the interacting partners with influence on IRES activity. These results indicate that PTB conveys a role in the regulation of the IRES activity of Cx55.5.
Conclusion
Our findings indicate that the activity of the IRES element of the neuronal gap junction protein Cx55.5 is subject of regulation through flanking polypyrimidine tracts, and that the non-canonical trans-activation factor PTB plays an essential role in this process. This observation is of considerable importance and may provide initial insight into molecular-functional relationships of electrical coupling in horizontal cells.
doi:10.1186/1471-2199-9-92
PMCID: PMC2579433  PMID: 18947383
2.  IRES-mediated translation of the carboxy-terminal domain of the horizontal cell specific connexin Cx55.5 in vivo and in vitro 
Background
Changes of the interneuronal coupling mediated by electrical synapse proteins in response to light adaptation and receptive field shaping are a paramount feature in the photoreceptor/horizontal cell/bipolar cell (PRC/HC/BPC) complex of the outer retina. The regulation of these processes is not fully understood at the molecular level but they may require information transfer to the nucleus by locally generated messengers. Electrical synapse proteins may comprise a feasible molecular determinant in such an information-laden signalling pathway.
Results
Connexin55.5 (Cx55.5) is a connexin with horizontal cell-restricted expression in zebrafish accumulating at dendritic sites within the PRC/HC/BPC complex in form of hemichannels where light-dependent plasticity occurs. Here we provide evidence for the generation of a carboxy-terminal domain of Cx55.5. The protein product is translated from the Cx55.5 mRNA by internal translation initiation from an in-frame ATG codon involving a putative internal ribosome entry site (IRES) element localized in the coding region of Cx55.5. This protein product resembling an 11 kDa domain of Cx55.5 is partially located in the nucleus in vivo and in vitro.
Conclusion
Our results demonstrate the generation of a second protein from the coding region of Cx55.5 by an IRES mediated process. The nuclear occurrence of a fraction of this protein provides first evidence that this electrical synapse protein may participate in a putative cytoplasmic to nuclear signal transfer. This suggests that Cx55.5 could be involved in gene regulation making structural plasticity at the PRC/HC/BPC complex feasible.
doi:10.1186/1471-2199-9-52
PMCID: PMC2435236  PMID: 18505575

Results 1-2 (2)