Ligand-induced signaling in the absence of furin processing of Notch1

Notch is a conserved cell surface receptor that is activated through direct contact with neighboring ligand-expressing cells. The primary 300-kDa tran slation product of the Notch1 gene (p300) is cleaved by a furin-like conver tase to generate a heterodimeric, cell-surface receptor composed of 180- (p 180) and 120- (p120) kDa polypeptides. Heterodimeric Notch is thought to be the only form of the receptor which is both present on the cell surface an d able to generate an intracellular signal in response to ligand. Consisten t with previous reports, we found that disruption of furin processing of No tch1, either by coexpression of a furin inhibitor or by mutation of furin t arget sequences within Notch1 itself, perturbed ligand-dependent signaling through the well-characterized mediator of Notch signal transduction, CSL ( CBF1, Su(H), and LAG-1). Yet contrary to these reports, we could detect the full-length p300 Notch1 product on the cell surface. Moreover, this unclea ved form of Notch1 could suppress the differentiation of C2C12 myoblasts in response to ligand. Taken together, these data support our previous studie s characterizing a CSL-independent Notch signaling pathway and identify thi s uncleaved isoform of Notch as a potential mediator of this pathway. Our r esults suggest a novel paradigm in signal transduction, one in which two is oforms of the same cell-surface receptor could mediate two distinct signali ng pathways hi response to ligand. (C) 2001 Academic Press.