Before revision:  In this wrok, the 3-dimensional probability distribution of Ca^{2+}  which is read out by phosphorylation  is studied in two scenarios with simulation and mean field ansatz .
After revision:  In this work, with both numerical simulation and mean field ansatz, the 3-dimensional probability distribution of Ca^{2+} which is read out by phosphorylation is studied in two scenarios with simulation and mean field ansatz .