EXPRESSION OF THE HUMAN OXYTOCIN RECEPTOR IN BACULOVIRUS-INFECTED INSECT CELLS - HIGH-AFFINITY BINDING IS INDUCED BY A CHOLESTEROL CYCLODEXTRIN COMPLEX
G. Gimpl et al., EXPRESSION OF THE HUMAN OXYTOCIN RECEPTOR IN BACULOVIRUS-INFECTED INSECT CELLS - HIGH-AFFINITY BINDING IS INDUCED BY A CHOLESTEROL CYCLODEXTRIN COMPLEX, Biochemistry, 34(42), 1995, pp. 13794-13801
We have expressed a c-myc epitope-tagged human oxytocin receptor in th
e baculovirus/Sf9 cell system. The receptor was identified by SDS-PAGE
and subsequent immunoblot as a similar to 50 kDa protein which decrea
sed to about 44 kDa upon treatment with tunicamycin, Binding studies s
howed that the human oxytocin receptor was expressed in a low-affinity
state (K-d = 215 nM, B-max = 1.66 pmol/mg). After addition of cholest
erol in the form of a soluble cholesterol-methyl-beta-cyclodextrin com
plex to the membranes, we obtained part of the human oxytocin receptor
in its high-affinity state for oxytocin (K-d = 0.96 nM and B-max = 31
8 is fmol/mg of protein). In subsequent studies, we added the choleste
rol-methyl-beta-cyclodextrin complex to the Sf9 cell culture medium at
various times post infection. Binding analysis showed that this resul
ts in a more than 3-fold further increase in functional receptor bindi
ng sites of high-affinity state (B-max = 1.08 pmol/mg), The cholestero
l effect was dose-dependent, with an EC(50) of about 50 mu M cholester
ol. Due to these findings, we determined the cholesterol and phospholi
pid content in purified Sf9 plasma membranes. The untreated naturally
cholesterol auxotroph insect cells grown in medium with 2% fetal calf
serum had a molar cholesterol/phospholipid ratio of about 0.04, which
is approximately 20-fold lower than normally found in plasma membranes
of higher eukaryotic cells. The high-affinity binding of the oxytocin
receptor increased in parallel with the cholesterol levels present in
the corresponding plasma membranes, Here we show for the first time t
hat cholesterol can be a critical factor for the function of membrane
proteins expressed in the baculovirus/Sf9 cell system.