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Fig. 6 | Theoretical Biology and Medical Modelling

Fig. 6

From: Steady state analysis of influx and transbilayer distribution of ergosterol in the yeast plasma membrane

Fig. 6

Bi-molecular complex formation of sterol with phospholipids gives rise to combined hyperbolic and linear sterol influx into the PM and the ER. Accounting explicitly for sterol complex formation with a limited number of phospholipids in the outer PM leaflet (PT1 = 300 nM), the inner PM leaflet (PT2 = 1000 nM) or the ER (PT1 = 600 nM) results in a bi-phasic dependence of sterol abundance in each membrane pool on sterol influx (v1, varied from 0.5 pM∙s− 1 to 0.33 nM∙s− 1). The highest flux value is 10times that calculated for yeast cells using realistic values of catalytic activity and abundance of the sterol importers Aus1/Pdr11 (see text). Thus, it would represent a 10fold increased expression level of Aus1/Pdr11. Sterol abundance was calculated using Eqs. 4850 and 5254 for different values of the equilibrium constant for sterol complexation in the outer PM leaflet, q4, and plotted as function of sterol in flux for the PM (a) and for total cellular sterol (c). For the ER pool (which is independent of q4, see Eqs. 50 and 54), the free (‘active’; black line in b), complexed (red line in b) and total sterol (green line in b) was plotted as function of sterol influx. Other parameters were k2 = 0.1 s− 1, k3 = 0.01 s− 1, q5 = 1.0 and q6 = 0.5

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