Figure 5.

Simplified membrane interaction model sets constraints on the strength of basic residue attraction and phosphorylation-driven repulsion from membrane. (A) Schematic diagrams reflecting probability density of tyrosines under different conditions are given. Based on previous experimental studies, our model must be consistent with the following: probability density of tyrosines is narrow and close to the membrane for wild-type ɛ but widens and shifts away from the membrane when it is phosphorylated or the basic residues are mutated. (B) Interaction potentials used in the simplified model are shown. Basic residues experience an attractive potential of depth E_B0 near the membrane and experience zero potential one Debye length (∼1 nm) away from the membrane (blue solid line). Phosphorylated tyrosines experience a repulsive potential with strength E_P0 (red dashed line). Unphosphorylated tyrosines and all other amino acids are hindered from entering the membrane but otherwise experience no potential (green dotted line). (C) Probability density of the distance from the membrane of the first tyrosine of ɛ, assuming no phosphorylated tyrosines, is shown for varying strengths of the basic residue potential, E_B0 (white: low; black: high). The tyrosine moves close to the membrane as E_B0 is increased. Probability density when there is no basic residue potential (E_B0 = 0 k_BT) is shown as blue dotted line. (Inset) Variance of probability density of first tyrosine over range of basic residue potential strengths is shown. Green dashed line shows the characteristic E_B0 = 0.5k_BT required to confine the tyrosine to the membrane, defined in the text. (D) Probability density of the location of first tyrosine assuming all tyrosines are phosphorylated is shown for E_B0 = 0.5k_BT (the value required to confine the tyrosine to the membrane assuming no tyrosines are phosphorylated) and varying phosphorylated tyrosine potential strength, E_P0 (low: white, high: black). Probability density when E_P0 = 2k_BT is shown as red dashed line, reflecting the E_P0-value needed to approximately return to the distribution when E_B0 = 0k_BT (blue dashed line). To see this figure in color, go online.