Two functionally distinct forms of NKX2.1 protein are expressed in the pulmonary epithelium

The homeodomain transcriptional factor NKX2.1 is critical for normal morphogenesis of the lung, thyroid, and the brain. In the lung, NKX2. 1 binds to and activates the expression of pulmonary differentiation-specific genes SP-A, SP-B, and SP-C. The Nkx2.1 gene is comprised of three exons separated by two introns. In both thyroid and lung, the predominant Nkx2.1 transcript includes exons II and III and is translated into a 371 amino acid protein. A minor transcript also exists which includes all three exons. This transcript encodes a 401 amino acid isoform of NKX2.1. The 30 amino acid extension is highly conserved amongst various mammalian species. In the current study, we demonstrate that the two NKX2.1 isoforms are functionally distinct and their corresponding transcripts are expressed differentially during mouse embryonic lung development. The results demonstrate that the longer isoform of NKX2.1 exhibits reduced activity in transactivating an SP-C target promoter when compared to the truncated major NKX2.1 protein. Site directed mutagenesis of the 30 amino acid peptide extension suggests that this fragment alters the activity of 5E likely by steric interference.