Strand-swap and X dimerizations have different calcium requirements. (A) Live-imaging of the calcium switch experiment with cells expressing the parental mutant and its strand-swap–incompetent W2A variant. A calcium switch was performed as in Fig 4 A, but only with different calcium concentration in high-calcium media (indicated). Note that the parental mutant forms clusters at 0.1 mM calcium. The W2A mutant forms clusters at much higher calcium concentrations. Bars, 10 µM. (B and C) Clustering kinetics of the parental mutant (B) and its W2A variant (C) at different calcium concentrations. The error bars indicate SD.