Involvement of calreticulin in signal transduction pathways. Calreticulin is essential for integrin signaling and the dephosphorylation and nuclear translocation of NF-AT3. The size of the Ca²⁺ symbols reflects the differing concentrations in the extracellular space and the sarcoplasmic reticulum (10^–3 M), as compared with the cytoplasm (10^–5 M in systole and 10^–7 M in diastole, with higher levels seen in Ca²⁺ sparks).