T cell receptor phosphorylation by Lck is an essential step in T cell activation. It is known that the conformational states of Lck control enzymatic activity; however, the underlying principles of how Lck finds its substrate over the plasma membrane remain elusive. Here, single-particle tracking is paired with photoactivatable localization microscopy to observe the diffusive modes of Lck in the plasma membrane. Individual Lck molecules switched between free and confined diffusion in both resting and stimulated T cells. Lck mutants locked in the open conformation were more confined than Lck mutants in the closed conformation. Further confinement of kinase-dead versions of Lck suggests that Lck confinement was not caused by phosphorylated substrates. Our data support a model in which confined diffusion of open Lck results in high local phosphorylation rates, and inactive, closed Lck diffuses freely to enable long-range distribution over the plasma membrane.
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