T cells exhibit near single-molecule sensitivity to foreign agonist, and misregulation in T cell signaling leads to immune diseases. It has been known for decades that the scaffold protein LAT (linker for activation in T cell) forms extended protein assemblies (originally referred to as LAT microclusters) upon phosphorylation by kinases on activated T cell receptor (TCR). More recently, it has been demonstrated that LAT molecules, along with other signaling proteins, condense abruptly into discrete condensates in response to individual molecular TCR activation events. The condensation dynamics suggests a type of kinetic phase transition, rooted in the balance of kinase and phosphatase reactions. Here, we seek to resolve the earliest nucleation events in the LAT condensation process. As freshly phosphorylated LATs begin to be crosslinked, their molecular mobility in the plasma membrane will be reduced. Here, I will discuss our recent observations of theses prenucleation clusters of LAT using a type of single molecule displacement imaging. These results indicate a clear distinction between small, prenucleation clusters, and mature LAT condensates that have been resolved by other methods.
Choi et al. (Sun,) studied this question.