The Notch receptor is a key component of a highly conserved signaling pathway that regulates cell fate determination during development. In Drosophila, where Notch signaling was first identified and studied, there is only one Notch receptor. In contrast, mammals have four Notch receptor genes, Notch1-4. Notch1 and Notch2 are both required for embryo viability, are widely expressed in mammals, and are structurally conserved. It is presently unknown if these two receptors are functionally redundant or if they have unique capabilities related to differences in their amino acid sequences. In contrast to the rest of the molecule, the amino acid sequences of a large region of the Notch intracellular domain are not highly conserved and thus may be able to interact with distinct transcription factors and mediate the expression of different sets of genes. To determine if the function of this region is conserved, the last 426 amino acids of the Notch2 receptor have been replaced with the corresponding region of Notch1 in mice by using gene targeting. We have determined that even though the amino acid sequences of this region are only 37% identical (137/426), the C-terminal region of the Notch1 intracellular domain can functionally replace that of Notch2 in vivo.
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