Structural recognition of an ICAM-1 peptide by its receptor on the surfaceof T cells: conformational studies of cyclo (1,12)-Pen-Pro-Arg-Gly-Gly-Ser-Val-Leu-Val-Thr-Gly-Cys-OH

The purpose of this study is to elucidate the solution conformation of cycl ic peptide 1 (clBR), cycle (1, 12)-Pen1-Pro2-Arg3-Gly4-Gly5-Ser6-Val7-Leu8- Val9-Thr10-Gly11-Cys12-OH, using NMR, circular dichroism (CD) and molecular dynamics (MD) simulation experiments, clBR peptide (1), which is derived f rom the sequence of intercellular adhesion molecule-1 (ICAM-1, CD54), inhib its homotypic T-cell adhesion in vitro. The peptide hinders T-cell adhesion by inhibiting the leukocyte function-associated antigen-1 (LFA-1, CD11a/CD 18) interaction with ICAM-1. Furthermore, Molt-3 T cells bind and internali ze this peptide via cell surface receptors such as LFA-1. Peptide internali zation by the LFA-1 receptor is one possible mechanism of inhibition of T-c ell adhesion. The recognition of the peptide by LFA-1 is due to its sequenc e and conformation; therefore, this study can provide a better understandin g for the conformational requirement of peptide-receptor interactions. The solution structure of 1 was determined using NMR, CD and MD simulation in a queous solution. NMR showed a major and a minor conformer due to the presen ce of cis/trans isomerization at the X-Pro peptide bond. Because the contri bution of the minor conformer is very small, this work is focused only on t he major conformer. In solution, the major conformer shows a trans-configur ation at the Pen1-Pro2 peptide bond as determined by HMQC NMR. The major co nformer shows possible beta-turns at Pro2-Arg3-Gly4-Gly5, Gly5-Ser6-Val7-Le u8, and Val9-Thr10-Gly11-Cys12. The first beta-turn is supported by the ROE connectivities between the NH of Gly4 and the NH of Gly5. The connectiviti es between the NH of Ser6 and the NH of Val7, followed by the interaction b etween the amide protons of Val7 and Leu8, support the presence of the seco nd beta-turn. Furthermore, the presence of a beta-turn at Val9-Thr10-Gly11- Cys12 is supported by the NH-NH connectivities between Thr10 and Gly11 and between Gly11 and Cys12. The propensity to form a type I beta-turn structur e is also supported by CD spectral analysis. The clBR peptide (1) shows str uctural similarity at residues Pro2 to Val7 with the same sequence in the X -ray structure of D1-domain of ICAM-1. The conformation of Pro2 to Val7 in this peptide may be important for its binding selectivity to the LFA-1 rece ptor.