Modulation of CTP:phosphocholine cytidylyltransferase by membrane curvature elastic stress

Abstract

The activity of CTP:phosphocholine cytidylyltransferase, a rate-limiting enzyme in phosphatidylcholine biosynthesis, is modulated by its interaction with lipid bilayers [Kent, C. (1997) Biochim. Biophys. Acta 1348, 79-90]. Its regulation is of central importance in the maintenance of membrane lipid homeostasis. Here we show evidence that the stored curvature elastic stress in the lipid membrane's monolayer modulates the activity of CTP:phosphocholine cytidylyltransferase. Our results show how a purely physical feedback signal could play a key role in the control of membrane lipid synthesis.

Figure 1

The CCT-binding helix is seen in cross section bound to the membrane. The geometry of the hydrophobic part of the binding domain (dark line) allows the nearby amphiphiles (gray background) to splay. The molecules are approximately to scale.

Figure 2

Composite plot showing the relative activity, S, of CCT as a function of the mol fraction of DOPC, x, in DMPC/DOPC (solid circles) and DOPC/DOPE (open circles) vesicles. The curve through the data points in the DOPC/DOPE part of the plot is obtained from Eq. 5.

Figure 3

Plot showing the effect of lyso-MPC (open circles), HexPC (open squares) and C₁₆EO₈ (solid circles) as a function of their fractional molar composition in DOPC/DOPE vesicles (9:1 mol ratio) on the relative activity of CCT.

Figure 4

How the cell might regulate membrane composition can be seen from the effect of CCT binding on the torque tension of a DOPE and DOPC monolayer. (a) A curvature relaxed DOPE monolayer is flattened, b, with a resultant increase in torque tension, indicated by the dark shading in the chain region. (c) The partitioning of the helical CCT-binding domain into the monolayer allows DOPE molecules close by to splay and hence reduce their torque tension. (d) DOPC has a lower propensity for interfacial curvature and hence when it is flattened, e, the torque tension is less than a third of that of DOPE, as indicated by the lighter shading. (f) Hence the energy released on binding CCT is less than for DOPE, and fewer CCT molecules partition into the monolayer.