New work shows that activation of the Ras guanine nucleotide exchange factor SOS is dependent upon the membrane density of phosphatidylinositol-4,5-bisphosphate (PIP 2) and GTP-bound Ras.

This takes place when molecules diffuse across the cell membrane by travelling through specific transport proteins. It occurs down a concentration gradient - molecules move from an area of high to ...

"These changes make the apical membrane—the top of the cell—less stiff and certain constituents of the tight junctions more mobile, without however affecting the barrier formed by these ...