Structure-Function analysis of the CTLA-4 interaction with PP2A
Wendy A Teft1, Thu A Chau, Joaquín Madrenas
1FOCIS Centre for Clinical Immunology and Immunotherapeutics, Robarts Research Institute, The University of Western Ontario, London, Ontario, Canada. wateft@uwo.ca
Background:
CTLA-4 functions primarily as an inhibitor of T cell activation. There are several candidate explanations as to how CTLA-4 modulates T cell responses, but the exact mechanism remains undefined. The tail of CTLA-4 does not have any intrinsic enzymatic activity but is able to associate with several signaling molecules including the serine/threonine phosphatase PP2A. PP2A is a heterotrimeric molecule comprised of a regulatory B subunit associated with a core dimer of a scaffolding (A) and a catalytic (C) subunit.
Results:
Here, we performed an analysis of the human CTLA-4 interface interacting with PP2A. We show that PP2A interacts with the cytoplasmic tail of CTLA-4 in two different sites, one on the lysine rich motif, and the other on the tyrosine residue located at position 182 (but not the tyrosine 165 of the YVKM motif). Although the interaction between CTLA-4 and PP2A was not required for inhibition of T cell responses, it was important for T cell activation by inverse agonists of CTLA-4. Such an interaction was functionally relevant because the inverse agonists induced IL-2 production in an okadaic acid-dependent manner.
Conclusion:
Our studies demonstrate that PP2A interacts with the cytoplasmic tail of human CTLA-4 through two motifs, the lysine rich motif centered at lysine 155 and the tyrosine residue 182. This interaction and the phosphatase activity of PP2A are important for CTLA-4-mediated T cell activation.
Insights
Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) interacts with protein phosphatase 2A (PP2A) via its tail, influencing T cell activation. This interaction is crucial for inverse agonist-mediated T cell responses.
Area of Science:
- Immunology
- Molecular Biology
- Cell Signaling
Background:
- CTLA-4 is a key inhibitor of T cell activation, but its precise mechanism of action is not fully understood.
- The cytoplasmic tail of CTLA-4 lacks intrinsic enzymatic activity but associates with signaling molecules like protein phosphatase 2A (PP2A).
- PP2A is a heterotrimeric enzyme composed of scaffolding (A), catalytic (C), and regulatory (B) subunits.
Purpose of the Study:
- To investigate the interaction interface between human CTLA-4 and PP2A.
- To determine the functional significance of the CTLA-4/PP2A interaction in T cell responses.
- To elucidate the role of PP2A phosphatase activity in CTLA-4-mediated T cell modulation.
Main Methods:
- Analysis of the human CTLA-4 cytoplasmic tail interaction with PP2A.
- Site-directed mutagenesis to identify key interaction residues.
- Assays to measure T cell activation and IL-2 production.
Main Results:
- PP2A binds to the CTLA-4 cytoplasmic tail at two distinct sites: a lysine-rich motif and tyrosine residue 182.
- This interaction is not essential for CTLA-4's inhibitory function but is critical for T cell activation induced by CTLA-4 inverse agonists.
- Inverse agonists stimulated IL-2 production in an okadaic acid-dependent manner, highlighting the role of PP2A activity.
Conclusions:
- PP2A directly interacts with the cytoplasmic tail of human CTLA-4 through specific motifs (lysine-rich motif at K155 and Y182).
- This CTLA-4/PP2A interaction, along with PP2A's phosphatase activity, plays a significant role in CTLA-4-mediated T cell activation.
- The findings provide new insights into the molecular mechanisms underlying CTLA-4 signaling in T cells.
Related Concept Videos
01:22T Cell Activation and Clonal Selection
Naive T cells that have not yet encountered an antigen express two primary CD...
01:23Catenins
Catenins in Cell Junctions
Catenins bind to cell adhesion molecules such as cadherins and link them to different cytoskeletal proteins depending on the type of cell junction. At the adherens...
01:19Allosteric Proteins-ATCase
Aspartate transcarbamoylase (ATCase) is a cytosolic enzyme that catalyzes the condensation of L-aspartate and carbamoyl phosphate to N-carbamoyl-L-aspartate. This reaction is the first step in pyrimidine biosynthesis. UTP and CTP, the end products of the pyrimidine synthesis pathway,...