A Ca2+ binding cyclic peptide derived from the alpha-subunit of LFA-1: inhibitor of ICAM-1/LFA-1-mediated T-cell adhesion.

The objective of this work is to study the conformation of cyclic peptide (1), cyclo (1, 12) Pen1-Gly2-Val3-Asp4-Val5-Asp6-Gln7-+ ++Asp8-Gly9-Glu10-Thr11-Cys12, in the presence and absence of calcium. Cyclic peptide 1 is derived from the divalent cation binding sequence of the alpha-subunit of LFA-1. This peptide has been shown to inhibit ICAM-1-LFA-1 mediated T-cell adhesion. In order to understand the structural requirements for this biologically active peptide, its solution structure was studied by nuclear magnetic resonance (NMR), circular dichroism (CD) and molecular dynamics simulations. This cyclic peptide exhibits two types of possible conformations in solution. Structure I is a loop-turn-loop type of structure, which is suitable to bind cations such as EF hand proteins. Structure II is a more extended structure with beta-hairpin bend at Asp4-Val5-Asp6-Gln7. There is evidence that alterations in the conformation of LFA-1 upon binding to divalent cations cause LFA-1 to bind to ICAM-1. To understand this mechanism, the cation-binding properties of the peptide were studied by CD and NMR. CD studies indicated that the peptide binds to calcium and forms a 1 : 1 (peptide: calcium) complex at low calcium concentrations and multiple types of complexes at higher cation concentrations. NMR studies indicated that the conformation of the peptide is not significantly altered upon binding to calcium. The peptide can inhibit T-cell adhesion by directly binding to ICAM-1 or by disrupting the interaction of the alpha and beta-subunits of LFA-1 protein. This study will help us to understand the mechanism(s) of action of this peptide and will improve our ability to design a better inhibitor of T-cell adhesion.