Living cells respond to spatially confined signals. Intracellular signal transmission often involves the release of second messengers like Ca2+. They eventually trigger a physiological response, for example, by activating kinases that in turn activate target proteins through phosphorylation. Here, we investigate theoretically how positional information can be accurately read out by protein phosphorylation in spite of rapid second messenger diffusion. We find that accuracy is increased by binding of kinases to the cell membrane prior to phosphorylation and by increasing the rate of Ca2+ loss from the cell interior. These findings could explain some salient features of the conventional protein kinase Cα.