I am trying to use sortase to ligate fluorescent LPxTG probe onto the surface of HEK 293 cells, where glycine motifs have been incorporated. However the cells seem to die when incubated for 30 min on ice in the sortase ligation buffer, and I can't see any fluorescent labelling on the flow cytometer. The ligation buffer is 50 mM HEPES, 150 mM NaCl, 5 mM CaCl2. Should HEK cells be able to tolerate this, in particular the calcium? The sortase requires calcium as a cofactor. Or do I need to change the ligation buffer?