Cooperative assembly of a four-molecule signaling complex formed upon T cell antigen receptor activation

Significance An early event during cellular activation in many biologic systems is recruitment of the cytosolic enzyme phospholipase-Cγ1 to the plasma membrane where, on activation, it cleaves its lipid substrates into functional second messengers. T cell antigen receptor engagement during the immune response leads to rapid formation of a multiprotein complex that brings phospholipase-Cγ1 to the plasma membrane with three adapter molecules. We reconstituted this quaternary complex in vitro and used biophysical techniques to determine stoichiometry and measure the affinities of the interacting proteins and the energetics of formation. We observe cooperative formation of a circular loop of interactions associated with a significant entropic penalty facilitating reversibility. Such instability might be a target of regulation during T cell activation.