This demo shows the behavior of a slightly different model of the alpha pathway receptor complex. In this version, the membrane-spanning receptor protein Ste2 is bound directly to Gpa1. Ste2 effects the conformational change in Gpa1 that results in nucleotide exchange via their direct interface, instead of through Ste4.
The result is a slightly different profile in progress of the nucleotide exchange reaction.
The main plot shows relatively little qualitative difference from the form of the simulation in which the receptor Ste2 only communictes with Gpa1 through Ste4. The red "four-one-total" trace of all species containing Ste4 and Gpa1 coincides with the dark green trace labeled "four-one-GTP." In this simulation Gpa1 is always bound to either GTP or GDP. You can see by looking at the blue "four-one-GTP" trace of all species containing Ste4 and Gpa1 with Gpa1 bound to GTP, essentially none of this GTP-bound Gpa1 remains bound to Ste4; hence the coincidence of the red and dark green traces.
The interesting action happens at time 2.0, when alpha factor is introduced. The unbinding of Gpa1 from Ste4 is seen in how the dark-green trace falls at this time. The binding of Ste5 to Ste4 permitted by Gpa1's abandoning Ste4 is seen in the rise of the purple "four-five" trace of all species of complexes containing Ste4 and Ste5 bound together.
The next trace shows the profile of the nucleotide exchange reaction. It differs somewhat from the corresponding plot in the other form of the receptor complex, in which the conformational change effected by Ste2 on Gpa1 must be communicated through Ste4.
