Parameters used in the simulations and the equilibrium model calculations.

This table furnishes values of parameters used in the simulations and the equilibrium model calculations. is a negative cooperativity factor for the binding of a second Grb2 to a Grb2-SOS1 complex (see Eq. (1)) [6]. The equilibrium constants for the binding of Grb2 to one of the three terminal phosphotyrosines on LAT range from M[6], [33], [45]. In our model the affinities for these three binding sites are identical. We take M. The values for and are from biacore studies of the binding of the SH2 domain in a Grb2-SOS1 complex to an eleven peptide sequence from the cytoplasmic domain of EGFR that includes the Grb2 binding site pY1068 [48]. The values of these rate constants have no effect on any of the equilibrium results. The value of the equilibrium crosslinking constant is estimated from the value as in Nag et al. (2009). The rate constant is calculated using the values of and which we assume equals . The value of is taken from [49]. The dissociation constants for the binding of the Grb2 SH3 domain to the N-terminal and C-terminal proline-rich regions of SOS1 are 260 and 510 respectively [6]. In our model we do not distinguish between the two SH3 binding sites on SOS1 and take , the geometric mean of the two values. The value of is calculated using the value of and the geometric mean of the values. The value of is estimated using Eq. (3) and taking . The diameter of the Jurkat cell, , has been measured by Rosenbluth et al. [50] to be 11.5 with the cytosol taking up about of the total cell volume. is the cytosolic volume. The surface area of the Jurkat cell is taken to be approximately twice the area of a sphere of diameter 11.5 . In the stochastic simulations, only unimolecular rate constants can be used directly so the solution bimolecular rate constants are scaled by the cytosolic volume and the surface bimolecular rate constants are scaled by . The exact values of the parameters we used are given in the table to allow our results to be reproduced. The accuracy of the parameters that have been determined by experiment is at best two significant figures.