If Numb functions to block Notch signaling, as is suspected, then the ubiquitous Notch distribution found is consonant with its proposed function. To assess the possibility of a direct physical interaction between Notch and Numb, a yeast two hybrid interaction assay has been carried out. In this experiment genes coding for fragments of each protein are placed in yeast cells, one attached to a coding sequence specifying a DNA binding protein, and the other attached to a coding sequence specifying a transcriptional activator domain. The N-terminal RAM23 domain does indeed interact with Numb in the yeast two hybrid experiment. Since Numb can interact with the intracellular domain of Notch, it is presumbed that Numb can interfer with Notch signaling.
Experimental evidence suggests there is.
In Notch mutants, extra neurons have been detected, resulting from a transformation of sheath cells into neurons. Thus Notch function is required not only to specify the sheath cell but also to express ttk in this daughter cell of an asymmetric division. In a reciprocal experiment, overexpression of Notch turns on ttk expression in cells that normally do not express ttk . Both maternal and zygotic forms have multiple PEST sequences, correlating with rapid protein turnover. Numb protein has an N-terminal phosphotyrosine binding domain.