Homooligomerization of the Cytoplasmic Domain of the T Cell Receptor ζ Chain and of Other Proteins Containing the Immunoreceptor Tyrosine-Based Activation Motif

Abstract

Antigen receptors on T cells, B cells, mast cells, and basophils all have cytoplasmic domains containing one or more copies of an immunoreceptor tyrosine-based activation motif (ITAM), tyrosine residues of which are phosphorylated upon receptor engagement in an early and obligatory event in the signaling cascade. How clustering of receptor extracellular domains leads to phosphorylation of cytoplasmic domain ITAMs is not known, and little structural or biochemical information is available for the ITAM-containing cytoplasmic domains. Here we investigate the conformation and oligomeric state of several immune receptor cytoplasmic domains, using purified recombinant proteins and a variety of biophysical and biochemical techniques. We show that all of the cytoplasmic domains of ITAM-containing signaling subunits studied are oligomeric in solution, namely, T cell antigen receptor ζ, CD3ε, CD3δ, and CD3γ, B cell antigen receptor Igα and Igβ, and Fc receptor FcεRIγ. For ζ_cyt, the oligomerization behavior is best described by a two-step monomer−dimer−tetramer fast dynamic equilibrium with dissociation constants in the order of ∼10 μM (monomer−dimer) and ∼1 mM (dimer−tetramer). In contrast to the other ITAM-containing proteins, Igα_cyt forms stable dimers and tetramers even below 10 μM. Circular dichroic analysis reveals the lack of stable ordered structure of the cytoplasmic domains studied, and oligomerization does not change the random-coil-like conformation observed. The random-coil nature of ζ_cyt was also confirmed by heteronuclear NMR. Phosphorylation of ζ_cyt and FcεRIγ_cyt does not significantly alter their oligomerization behavior. The implications of these results for transmembrane signaling and cellular activation by immune receptors are discussed.