The X-ray crystal structure of beta-ketoacyl [acyl carrier protein] synthase I.

The crystal structure of the fatty acid elongating enzyme beta-ketoacyl [acyl carrier protein] synthase I (KAS I) from Escherichia coli has been determined to 2.3 A resolution by molecular replacement using the recently solved crystal structure of KAS II as a search model. The crystal contains two independent dimers in the asymmetric unit. KAS I assumes the thiolase alpha(beta)alpha(beta)alpha fold. Electrostatic potential distribution reveals an acyl carrier protein docking site and a presumed substrate binding pocket was detected extending the active site. Both subunits contribute to each substrate binding site in the dimer.

Solvent exposed side chains of peptides bound to HLA A*1101 have similar effects on the reactivity of alloantibodies and specific TCR.

Peptides can affect the recognition of MHC class I molecules by allospecific antibodies. Two explanations have been proposed for this phenomenon. The 'conformational change' hypothesis suggests that peptide binding affects the availability of serologic determinants in the class I alpha1 and alpha2 domains while the 'peptide-side-chain effect' predicts that solvent exposed residues in the peptide are part of the serologic epitope. We have tested these possibilities by examining the recognition of peptide loaded HLA A*1101 molecules expressed in transporters associated with antigen processing (TAP)-deficient cell lines by three A11-specific mAb, and by comparing the effect of peptide analogues on the recognition of A11 complexes containing peptide epitopes from the Epstein-Barr virus nuclear antigen EBNA4 by antibodies and cytotoxic T lymphocytes (CT). The AUF5.13 and HB164 antibodies showed selective recognition of A11 molecules bound to partially overlapping sets of peptides from viral or cellular origin. The peptide dependence of AUF5.13 was confirmed in reconstitution experiments where A11 molecules were refolded at the surface of TAP-deficient T2/A11 cells that had been cultured at 26 degrees C and treated at pH3. Molecular modelling and Ala scanning mutagenesis of the IVTDFSVIK (IVT) and AVFDRKSDAK (AVF) peptides demonstrated that solvent-exposed peptide side chains affect CTL recognition as well as antibody binding. Substitution of Phe-P5 or Ser-P6 of the IVT peptide with Arg or Lys inhibited AUF5.13 recognition while binding was induced by substitution of the Arg-P5 and Lys-P6 of the AVF peptide with Ala. The results suggest that some allospecific antibodies recognized the surface of MHC class I-peptide complexes in a fashion similar to the TCR. This may involve direct interaction with the peptide side chains as well as recognition of peptide-induced perturbations in the class I complex.