J. Sakai et al., STEROL-REGULATED RELEASE OF SREBP-2 FROM CELL-MEMBRANES REQUIRES 2 SEQUENTIAL CLEAVAGES, ONE WITHIN A TRANSMEMBRANE SEGMENT, Cell, 85(7), 1996, pp. 1037-1046
Sterol regulatory element binding proteins (SREBPs) are transcription
factors attached to the endoplasmic reticulum. The NH2-segment, which
activates transcription, is connected to membranes by a hairpin anchor
formed by two transmembrane sequences and a short lumenal loop. Using
H-Ras-SREBP-2 fusion proteins, we show that the NH2-segment is releas
ed from membranes by two sequential cleavages. The first, regulated by
sterols, occurs in the lumenal loop. The second, not regulated by ste
rols, occurs within the first transmembrane domain. The liberated NH2-
segment enters the nucleus and activates genes controlling cholesterol
synthesis and uptake. Certain mutant Chinese hamster ovary cells are
auxotrophic for cholesterol because they fail to carry out the second
cleavage; the NH2-segment remains membrane-bound and transcription is
not activated.