RECURRENT G-TO-A SUBSTITUTION IN A SINGLE CODON OF SREBP CLEAVAGE-ACTIVATING PROTEIN CAUSES STEROL RESISTANCE IN 3 MUTANT CHINESE-HAMSTER OVARY CELL-LINES
A. Nohturfft et al., RECURRENT G-TO-A SUBSTITUTION IN A SINGLE CODON OF SREBP CLEAVAGE-ACTIVATING PROTEIN CAUSES STEROL RESISTANCE IN 3 MUTANT CHINESE-HAMSTER OVARY CELL-LINES, Proceedings of the National Academy of Sciences of the United Statesof America, 93(24), 1996, pp. 13709-13714
Oxygenated sterols such as 25-hydroxycholesterol kill Chinese hamster
ovary cells because they inhibit the proteolytic processing of sterol
regulatory element binding proteins (SREBPs), a pair of membrane-bound
transcription factors that activate genes controlling cholesterol syn
thesis and uptake from lipoproteins, The unprocessed SREBPs remain mem
brane-bound, they cannot activate the cholesterol biosynthetic pathway
, and the cells die of cholesterol deprivation. Several sterol-resista
nt hamster cell lines have been isolated previously by chemical mutage
nesis and selection for resistance to killing by 25-hydroxycholesterol
. We recently identified the defect in one such cell line (25-RA cells
) as a point mutation in a newly discovered membrane protein of 1276 a
mino acids, designated SREBP cleavage-activating protein (SCAP), The m
utation in the 25-RA cells resulted from a G-to-A transition in codon
443 of the SCAP gene, changing aspartic acid to asparagine. Wild-type
SCAP, when overexpressed by transfection, stimulates the proteolytic p
rocessing of both SREBPs, The D443N substitution is an activating muta
tion that increases the activity of SCAP and renders it resistant to i
nhibition by 25-hydroxycholesterol. We here report the identical G-to-
A transition in two additional lines of Chinese hamster ovary cells th
at were mutagenized and isolated by a similar protocol, The three muta
tions occurred independently as indicated by haplotype analysis of the
mutant genes using two intragenic sequence polymorphisms. All three c
ell lines were mutagenized with alkylating agents (nitrosoethylurea or
ethylmethane sulfonate) that favor Gto-A transitions. Nevertheless, t
he finding of the same nucleotide substitution at the same location in
all three cell lines indicates that SCAP may be unique in its ability
to stimulate SREBP cleavage, and residue 433 is a crucial determinant
of the protein's ability to be inhibited by 25-hydroxycholesterol.