In Vivo Consequences of Disrupting SH3-Mediated Interactions of the Inducible T-Cell Kinase

Noncanonical protein kinase A activation by oligomerization of regulatory subunits as revealed by inherited Carney complex mutations

Familial mutations of the protein kinase A (PKA) R1α regulatory subunit lead to a generalized predisposition for a wide range of tumors, from pituitary adenomas to pancreatic and liver cancers, commonly referred to as Carney complex (CNC). CNC mutations are known to cause overactivation of PKA, but the molecular mechanisms underlying such kinase overactivity are not fully understood in the context of the canonical cAMP-dependent activation of PKA. Here, we show that oligomerization-induced sequestration of R1α from the catalytic subunit of PKA (C) is a viable mechanism of PKA activation that can explain the CNC phenotype. Our investigations focus on comparative analyses at the level of structure, unfolding, aggregation, and kinase inhibition profiles of wild-type (wt) PKA R1α, the A211D and G287W CNC mutants, as well as the cognate acrodysostosis type 1 (ACRDYS1) mutations A211T and G287E. The latter exhibit a phenotype opposite to CNC with suboptimal PKA activation compared with wt. Overall, our results show that CNC mutations not only perturb the classical cAMP-dependent allosteric activation pathway of PKA, but also amplify significantly more than the cognate ACRDYS1 mutations nonclassical and previously unappreciated activation pathways, such as oligomerization-induced losses of the PKA R1α inhibitory function.

T Cell Costimulation by CD6 Is Dependent on Bivalent Binding of a GADS/SLP-76 Complex

The cell surface receptor CD6 regulates T cell activation in both activating and inhibitory manners. The adaptor protein SLP-76 is recruited to the phosphorylated CD6 cytoplasmic Y662 residue during T cell activation, providing an activating signal to T cells. In this study, a biochemical approach identified the SH2 domain-containing adaptor protein GADS as the dominant interaction partner for the CD6 cytoplasmic Y629 residue. Functional experiments in human Jurkat and primary T cells showed that both mutations Y629F and Y662F abolished costimulation by CD6. In addition, a restraint on T cell activation by CD6 was revealed in primary T cells expressing CD6 mutated at Y629 and Y662. These data are consistent with a model in which bivalent recruitment of a GADS/SLP-76 complex is required for costimulation by CD6.

Multimolecular analysis of stable immunological synapses reveals sustained recruitment and sequential assembly of signaling clusters

The formation of the immunological synapse between T cells and antigen-presenting cells (APC) begins within minutes of contact and can take hours for full T-cell activation. Although early phases of the synapse have been extensively studied for a select number of proteins, later phases have not yet been examined in detail. We studied the signaling network in stable synapses by measuring the simultaneous localization of 25 signaling and structural molecules over 2 h at the level of individual synapses using multi-epitope ligand cartography (MELC). Signaling proteins including phospho(p)ZAP70, pSLP76, pCD3ζ, and pLAT, along with proteins that influence synapse structure such as F-actin, tubulin, CD45, and ICAM-1, were localized in images of synapses and revealed the multidimensional construction of a mature synapse. The construction of the stable synapse included intense early TCR signaling, a phase of recruitment of structural proteins, and a sustained increase in signaling molecules and colocalization of TCR and pLAT signaling clusters in the center of the synapse. Consolidation of TCR and associated proteins resulted in formation of a small number of discrete synaptic microclusters. Development of synapses and cSMAC composition was greatly affected by the absence of Vav1, with an associated loss in PLCγ1 recruitment, pSLP76, and increased CXCR4. Together, these data demonstrate the use of multi-epitope ligand cartography to quantitatively analyze synapse formation and reveal successive recruitment of structural and signaling proteins and sustained phosphorylation at the mature synapse.

A conserved motif in the ITK PH-domain is required for phosphoinositide binding and TCR signaling but dispensable for adaptor protein interactions

Binding of the membrane phospholipid phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) to the Pleckstrin Homology (PH) domain of the Tec family protein tyrosine kinase, Inducible T cell Kinase (ITK), is critical for the recruitment of the kinase to the plasma membrane and its co-localization with the TCR-CD3 molecular complex. Three aromatic residues, termed the FYF motif, located in the inner walls of the phospholipid-binding pocket of the ITK PH domain, are conserved in the PH domains of all Tec kinases, but not in other PH-domain containing proteins, suggesting an important function of the FYF motif in the Tec kinase family. However, the biological significance of the FYF amino acid motif in the ITK-PH domain is unknown. To elucidate it, we have tested the effects of a FYF triple mutant (F26S, Y90F, F92S), henceforth termed FYF-ITK mutant, on ITK function. We found that FYF triple mutation inhibits the TCR-induced production of IL-4 by impairing ITK binding to PIP(3), reducing ITK membrane recruitment, inducing conformational changes at the T cell-APC contact site, and compromising phosphorylation of ITK and subsequent phosphorylation of PLCγ(1). Interestingly, however, the FYF motif is dispensable for the interaction of ITK with two of its signaling partners, SLP-76 and LAT. Thus, the FYF mutation uncouples PIP(3)-mediated ITK membrane recruitment from the interactions of the kinase with key components of the TCR signalosome and abrogates ITK function in T cells.