Structural Overview

Determining the crystal structure of the complex made deeper insights about the complex possible.

Fig: A structural overview video of MS2-3C8-MBP-DR4-CD4 complex (TCR-pMHC-CD4)

  

                                                                                    

• The complex takes the shape of a pointed arch in which both TCR and CD4 are tilted. The top of the arch is formed by α2 and β2 domains of HLA-DR4 (MHC) and the D1 domain of CD4. 
• Both TCR and CD4 are not perpendicular to the cell surface. TCR forms a 60 degree angle with the membrane, and CD4, around 70 degrees. The angle between pMHC and CD4 is around 50 degrees (see figure; legend: green and blue: MHCαβ; magenta and purple: TCRαβ; pink: CD4).

.                                                    Figure: The angles within the TCR complex

• The detected structure of the pMHC-CD4 complex was superpositioned onto that of a complex between mouse I-Ak and wild-type human CD4. This gave a low RMS value, showing that maturation of CD4 did not affect its overall binding mode to MHC class II.

• CD4 engages MHC class II through its membrane-distal domain D1 at a concavity formed by CD4's α2 and β2 domains. (see image)

• Minor substitution mutations in affinity-matured CD4 improved shape complementarity as well as increaseing hydrophobic interactions without altering the overall structure. Moreover, CD4-contacting residues are invariant across human polygenic MHC HLA-DR, -DP, -DQ alleles and also in mouse alleles. From these results, the topology of CD4-MHC class II interaction observed in TCR-pMHC-CD4 structure would be maintained at all times, irrespective of any MHC molecule.

• CD4 molecule retains overall extended conformation in both bound to TCR-pMHC state and unbound state with some hinge-like variability at interdomain junctions. (Fig)

• It's found that there are no direct contacts between TCR and CD4 in the -3C8-MBP-DR4-CD4 complex. Instead, all the crystal contacts are between CD4 and TCR/HLA-DR4. Examination of crystal contacts reinforces this and show that there are crystal contacts between CD4 and TCR.

• However, it was found that rather than crystal contacts, rigidity is what keeps TCR and CD4 apart. Evidence for this rigidity comes from seven independent views of CD4 D1-D4, each containing two molecules per asymmetric unit.

• When bound and unbound CD4 structures were pairwise compared, it was seen that angles between each of their D2-D3, D1-D2 and D3-D4 didn't vary much from one another. This limited segmental flexibility of C4 explains the large spatial separation (at the cell surface) between TCR and CD4 D4. Also, this prevents large conformationl changes of the co-receptor upon assembly of TCR-pMHC-CD4 complex during T-cell activation. (See image below)

.                     Figure: Stereoview of CD4 in bound form to TCR complex overlayed onto the CD4 in unbound form. Yin Wang et. al (2012)