Phosphorylationdependant interaction between antigenic peptides and MHC class 1: a molecular basis f

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Phosphorylation-dependant interaction between
antigenic peptides and MHC class 1 a molecular
basis for the presentation of transformed
selfFiyaz Mohammed et al. Nature Immunology
Volume 9, No. 11, November 2008
www.pnas.org/content/100/15.cover-expansion

• P. Mistry O. Parsons A. Prashar K. Wright

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Aim

• Can phosphorylation increase peptide binding
  affinity for HLA-A2?
• Major histocompatibility complexes (MHC) play an
  important role in immunity.
• Situated on the cell surface
• Changes in normal cell
• functioning can lead to
• altered self to the
• immune system

Fig3-15 Immunology, 7ed. (Garland Science 2008)
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Introduction

• MHCs interact with CD8 and T cells, which leads
  to cytolytic activity
• Evidence peptides containing post-translational
  modification (PTM) contribute to the pool of
  MHC-bound peptides thus represent potential
  targets for T cell recognition.
• Phosphopeptide antigens therapeutic interest
  deregulation of protein kinase activity is one of
  the hallmarks of malignant transformation
• MHC class 1 bound phosphopeptides represent a
  new set of target antigens for cancer
  immunotherapy

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Unusual characteristics of HLA2-A2-bound
phosphopeptides

• 1) 68 of p-Ser/p-Thr at P4 alone
• 2) 62 had Arg/Lys at P1
• 3) In predicted HLA-A2 binding proteins, no bias
  for P4 phosphorylation

- Affinities of phosphopeptides greater - Most
notable in peptides phosphorylated at P4 - Arg
(R) at P1 also increased binding affinity -
Presence of PC anchor residues also increased
affinity
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Phosphate-mediated phosphopeptide and HLA-A2
contacts

• Normal MHC-peptide complex structure conserved,
  with extended conformation on peptide
• Stabilizing interactions conserved
• P4 p-Ser exposed to solvent, available for
  contact with TCR
• Electrostatic interactions between a1 domain and
  Arg65
• p-Ser stabilized by conserved H20 molecule
• H-bonds between p-Ser, P2 carbonyl P1 Arg
  residue

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Subdominant anchor interactions are structurally
suboptimal

• Dominant P2 Leu anchor residue fills B pocket
  optimally provides H-bond ints.

Compared to
Met Thr
Gln
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Energetic basis of phosphopeptide binding to
HLA-A2

• Phosphopeptide RQApSIELPSM.
• Phosphorylated peptide 150-fold ? affinity for
  HLA-A2 than nonphosphorylated. With only slightly
  ? binding to HLA-A2-R65A.
• Substitution of P1 R?A in phosphopeptide caused
  only 5-fold ? in affinity for HLA-A2. With much ?
  affinity for HLA-A2-R65A.
• Reorientation of pS moiety may ? energetic
  contribution of interaction, explaining modest
  effects of mutations of P1/HLA-A2

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Effect of the phosphate moiety on TCR
recognition

• Phosphate is close to CDR3a loop on TCR, suggests
  TCR directly recognises phosphate moiety.

• CDR3ß loop is adjacent to C-terminal central
  sections of peptide.
• Phosphorylation may affect TCR ints indirectly
  as constraint at P4 resulting in differences in
  P5-PC region.

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Discussion

• Fiyaz Mohammed et al. demonstrate entirely new
  method for MHC binding
• No insertion of amino acid into pocket in peptide
  binding cleft
• Instead, interactions occur between P1 side
  chain/P4 phosphate moiety and MHC surface
• Phosphate surface anchor
• Only canonical P1 arginine/lysine, P4 phosphate
  moiety demonstrates increase in peptide
  presentation
• Phosphorylation at other amino acids has no
  effect on peptide MHC presentation.

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Discussion

• Peptide phosphorylation at P4 increases
  presentation by MHCs
• Deregulated phosphorylationpresentation of
  peptide antigens not presented on normal cells.
• P4 phosphate moiety has several functions
• Models indicate that P4 phosphate moiety binds
  CDR3a loop of TCR
• Constrains structure of peptide from P5-PCleads
  to MHC presentation of novel phosphopeptides