Figure 2. Four signalling modes predicted from kinetic modelling of the trimeric G-protein cycle.

Receptor activation starts at zero time. Mode 1 is sustained production of high levels of βγ and Gα^GDP, but not Gα^GTP, upon receptor activation. Mode 2 is production of high levels of βγ and Gα^GTP. Mode 3 is a sustained production of high levels of βγ, with Gα^GTP produced transiently and then falling to low levels, while mode 4 stabilizes Gα^GTP at high levels after the original overshoot. The kinetic parameters in all four modes are: starting [Gα^GDPβγ]=500 nM, and starting [βγ], [Gα^GDP] and [Gα^GTP]=1 nM; K₂=500 nM; K₃=2 nM. Other parameters are: k_ass=0.001 nM⁻¹·s⁻¹ (modes 1, 3 and 4), and 0.01 nM⁻¹·s⁻¹ (mode 2); k_diss=15 s⁻¹ (mode 1), 20 s⁻¹ (mode 2) and 25 s⁻¹ (modes 3 and 4); k_hydr=15 s⁻¹ (mode 1) and 5 s⁻¹ (modes 2–4); [Rc*]=30 nM (modes 1 and 2), 100 nM (mode 3) and 60 nM (mode 4); [GAP]=25 nM (modes 1 and 4), 20 nM (mode 2) and 52 nM (mode 3).