Failure to cleave sterol regulatory element-binding proteins (SREBPs) causes cholesterol auxotrophy in Chinese hamster ovary cells with genetic absence of SREBP cleavage-activating protein

We describe a line of mutant Chinese hamster ovary cells, designated SRD-13A, that cannot cleave sterol regulatory element-binding proteins (SREBPs) at site 1, due to mutations in the gene encoding SREBP cleavage- activating protein (SCAP). The SRD-13A cells were obtained by two rounds of γ-irradiation followed first by selection for a deficiency of low density lipoprotein receptors and second for cholesterol auxotrophy. In the SRD-13A cells, the only detectable SCAP allele encodes a truncated nonfunctional protein. In the absence of SCAP, the site 1 protease fails to cleave SREBPs, and their transcriptionally active NH₂-terminal fragments cannot enter the nucleus. As a result, the cells manifest a marked reduction in the synthesis of cholesterol and its uptake from low density lipoproteins. The SRD-13A cells grow only when cholesterol is added to the culture medium. SREBP cleavage is restored and the cholesterol requirement is abolished when SRD- 13A cells are transfected with expression vectors encoding SCAP. These results provide formal proof that SCAP is essential for the cleavage of SREBPs at site 1.