A Conserved Hydrophobic Patch on Vβ Domains Revealed by TCRβ Chain Crystal Structures: Implications for Pre-TCR Dimerization

Jin-Huan Liu,Ellis L Reinherz,Hongmin Zhang,Bo Zhou,Robert J Mallis,Jia-Huai Wang,Qiang Chen

doi:10.3389/fimmu.2011.00005

Abstract

The αβ T cell receptor (TCR) is a multimeric complex whose β chain plays a crucial role in thymocyte development as well as antigen recognition by mature T lymphocytes. We report here crystal structures of individual β subunits, termed N15β (Vβ5.2Dβ2Jβ2.6Cβ2) and N30β (Vβ13Dβ1Jβ1.1Cβ2), derived from two αβ TCRs specific for the immunodominant vesicular stomatitis virus octapeptide (VSV-8) bound to the murine H-2Kb MHC class I molecule. The crystal packing of the N15β structure reveals a homodimer formed through two Vβ domains. The Vβ/Vβ module is topologically very similar to the Vα/Vβ module in the N15αβ heterodimer. By contrast, in the N30β structure, the Vβ domain’s external hydrophobic CFG face is covered by the neighboring molecule’s Cβ domain. In conjunction with systematic investigation of previously published TCR single-subunit structures, we identified several conserved residues forming a concave hydrophobic patch at the center of the CFG outer face of the Vβ and other V-type Ig-like domains. This hydrophobic patch is shielded from solvent exposure in the crystal packing, implying that it is unlikely to be thermodynamically stable if exposed on the thymocyte surface. Accordingly, we propose a dimeric pre-TCR model distinct from those suggested previously by others and discuss its functional and structural implications.

Highlights

The αβ T cell receptor (TCR) is a multimeric complex consisting of the antigen-binding αβ clonotypic heterodimer and six invariantCD3 subunit dimers (Sun et al, 2001 and references therein)
The DNA sequence of N15β and N30β constructs were terminated immediately after the cysteine of the respective TCRβ subunit involved in interchain disulfide formation with the cysteine itself mutated to serine
The superposition of the isolated N15β protein onto the β subunit of our previously published N15 αβTCR heterodimer (PDB code 1NFD) provides one of the first examples demonstrating that there are no significant conformational changes upon TCRβ pairing with TCRα to form an intact αβTCR

Summary

Introduction

The αβ T cell receptor (TCR) is a multimeric complex consisting of the antigen-binding αβ clonotypic heterodimer and six invariantCD3 subunit dimers (Sun et al, 2001 and references therein). The first major checkpoint is referred to as β-selection During this process TCRβ chain gene rearrangement occurs in the absence of TCRα chain gene rearrangements (Von Boehmer et al, 2003). Signaling through the pre-TCR terminates TCRβ locus rearrangement, rescues cells from apoptosis and induces massive proliferation This process eventually enables CD4−CD8− double-negative (DN) cells to differentiate into CD4+CD8+ double-positive (DP) cells, facilitating TCRα gene rearrangement and generating a large αβTCR repertoire (Von Boehmer et al, 2003). DP thymocytes undergo positive selection and negative selection following self–MHC interaction with their αβTCR In the former case, DP cells mature into single positive (SP) thymocytes and may egress into the periphery (Starr et al, 2003; Juang et al, 2010). Cell death occurs and those apoptotic thymocytes are purged from the T lineage repertoire (Gallegos and Bevan, 2006; Ashton-Rickardt, 2007; Griesemer et al, 2010)

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