A Forced Opening

Abstract

Biochemistry![Figure][1] CREDIT: N. L. STEPHENSON AND J. M. AVIS, PNAS ADVANCED ONLINE EDITION, 24 SEPTEMBER 2012, REPRODUCED WITH PERMISSION FROM PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES U.S.A. Notch proteins are transmembrane receptors that have major roles in the regulation of cell proliferation and development, and mutations in the Notch signaling pathway contribute to human diseases and cancer. When Notch proteins are bound by their ligands, which are themselves transmembrane proteins found on adjacent cells, endocytosis of the Notch extracellular domain into the ligand-bearing cell occurs, during which the Notch protein is cleaved, allowing the intracellular domain to be translocated to the nucleus, where it activates target genes. Stephenson and Avis used atomic force microscopy (AFM) to physically stretch a portion of the receptor called the Notch negative regulatory region, which contains the masked site of proteolysis. They could show through AFM measurements and molecular dynamics simulations that such a mechanical stimulus unfolded the protein to expose a site where protolytic cleavage occurred (and actually monitored the latter as release of the AFM tip). The events and forces measured were consistent with a favored model of Notch signaling in which forces of ligand endocytosis expose the otherwise buried cleavage site, leading to slicing of the receptor and initiation of signaling. Proc. Natl. Acad. Sci. U.S.A. 109 , 10.1073/pnas.1205788109 (2012). [1]: pending:yes