Alternative splicing of chicken fibronectin in embryos and in normal and transformed cells.

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Abstract
To study the alternative splicing of fibronectin during embryogenesis and oncogenic transformation, we isolated cDNA clones of chicken fibronectin. The partial amino acid sequence deduced from sequencing of these clones showed that, overall, chicken fibronectin is approximately 80% identical with mammalian fibronectins. However, two of the three known regions of alternative splicing differed from this average. The V region was significantly more divergent, and RNA from embryonic chicken liver showed a pattern of V exon splicing which was distinct from that seen in human or rat fibronectins. In contrast, the EIIIB segment was very highly conserved (96%). As in mammals, this segment and another (EIIIA) were alternatively spliced in a cell-type-specific fashion. EIIIA+ and EIIIB+ species were almost absent in liver but predominated in total embryo RNA at all times from 2.5 to 11 days postfertilization. We also examined the possible contributions of fibronectin splicing and integrin receptor expression to the loss of fibronectin on oncogenic transformation. We detected little change in fibronectin splicing, other than a slight increase in representation of EIIIB+ species in fibroblasts after transformation by Rous sarcoma virus. It was also established that the overall reduction in fibronectin mRNA level observed after transformation was not accompanied by a decrease in integrin mRNA levels, indicating that fibronectin and integrin receptors are not coordinately regulated at the transcriptional level.