Int-2: a member of the fibroblast growth factor family has different subcellular fates depending on the choice of initiation codon.
The int-2 gene, which encodes a member of the fibroblast growth factor family, was discovered as a protooncogene transcriptionally activated following proviral insertion into adjacent chromosomal DNA. Analyses of the synthesis and processing of the int-2 protein, using an SV40-based vector to express cloned cDNA, showed four major products in the size range 27.5-31.5 kd that were associated with the secretory pathway. Further experiments using a cell-free translation system programmed with int-2 cRNA revealed a larger N-terminally extended protein. Site-directed mutagenesis of possible initiation codons confirmed that the first in-frame AUG codon would specify the start of a protein that includes a signal peptide for transport into the endoplasmic reticulum. However, protein synthesis also initiates at an upstream CUG codon to yield a polypeptide extended at the N-terminus by 29 amino acids. Immunofluorescent staining showed that a substantial proportion of the CUG-initiated protein resides in the cell nucleus, while a truncated int-2, lacking both the N-terminal extension and the signal peptide, was exclusively nuclear. These observations suggest that a nuclear localisation signal occurs in the body of the int-2 molecule, but is only accessible to the nuclear transport system if entry to the secretory pathway is compromised. Thus, the choice of initiation codon changes the subcellular fate of the int-2 protein and provides the potential for a duality of function through alternative transport pathways.
Dickson
C
C
Laboratory of Viral Carcinogenesis, Imperial Cancer Research Fund, London, UK.
Acland
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P
D000818:Animals; D001692:Biological Transport; D002478:Cells, Cultured; D003062:Codon / Q000235:genetics*; D004252:DNA Mutational Analysis; D005346:Fibroblast Growth Factors / Q000235:genetics*; D005822:Genetic Vectors; D000882:Haplorhini; D005810:Multigene Family / Q000235:genetics; D016297:Mutagenesis, Site-Directed; D010442:Peptide Chain Initiation, Translational / Q000235:genetics*; D021382:Protein Sorting Signals / Q000235:genetics*; D013539:Simian virus 40; D014162:Transfection
D003062:Codon; D021382:Protein Sorting Signals; D005346:Fibroblast Growth Factors