3. It sits at an angle — and almost always the same one · Prof. Kwon's TCR-HLA and CD4/CD8, First Steps

In 1996 two labs solved crystal structures of a TCR-pMHC complex within months of each other, and something nobody had known turned up.

A TCR does not sit square on the groove — it sits at an angle

The groove is a valley between two α helices, with the fragment lying in the bottom of it. The TCR covers that valley — but neither parallel to the helices nor at right angles to them. Roughly 45 to 70 degrees across.

And the strange part is that the direction is almost always the same.

Which side covers what

TCR side	Covers (class I)	Covers (class II)
Vα (CDR1α·CDR2α)	α2 helix	β1 helix
Vβ (CDR1β·CDR2β)	α1 helix	α1 helix
CDR3α · CDR3β	the middle — over the fragment

Why that arrangement matters is obvious the moment you recall lesson 5 of /tcell.

🧬

CDR1 · CDR2

Already written in the germline gene. Fixed the moment a V gene is chosen. Nearly identical between people.

🎲

CDR3

Made fresh at the join. Close to random, different in every person and every cell.

So the pre-written loops touch the plate and the freshly made loops touch the fragment. The diagonal is what makes that division of labour possible.

Rotate the TCR. Leave the angle behind and CDR3 lifts off the fragment.

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Why that direction — two answers

① It is written in the germline

The CDR1 and CDR2 positions of TRAV and TRBV genes carry residues that match the MHC helices, and those residues are evolutionarily conserved. That sets the direction. Lesson 4 takes this up.

② The coreceptor forces it

CD8 grabs α3 and CD4 grabs β2. That anchor is on one side only, so bringing Lck near the TCR means the TCR has to lean that way. Lessons 5 and 13 take this up.

The field's current answer is both — and which one dominates is still not settled. Lesson 4 walks through that argument.

Alpha and beta chains are filed separately and tied together by complex.id.

Python Runs in your browser

# A TCR is two chains paired. Count the pairs recorded in this table.
import thla

d = thla.vdj()
paired = d[d["cid"] != "0"]
size = paired.groupby("cid")["gene"].nunique()
both = size[size == 2].index
print(f"of {len(d):,} rows, {len(paired):,} carry a pairing id")
print(f"cells with both an alpha and a beta chain: {len(both):,}")
print()
one = paired[paired["cid"] == both[0]].sort_values("gene")
print("one cell unfolded")
for _, r in one.iterrows():
    print(f"   {r['gene']}  {r['v']:<12}{r['cdr3']:<20}{r['j']}")
print(f"   fragment the two see together : {one['epitope'].iloc[0]}  ·  plate {one['mhc_a'].iloc[0]}")

🤔 Check yourself

What does a TCR's CDR3 loop mainly touch?

🤔 Check yourself

Why are CDR1 and CDR2 nearly the same in everybody?

🤔 Check yourself

What two explanations are offered for the near-constant docking polarity?