Abstract
Previous studies have suggested that thrombin interacts with integrins in endothelial cells through its RGD (Arg-187, Gly-188, Asp-189) sequence. All existing crystal structures of thrombin show that most of this sequence is buried under the 220-loop and therefore interaction via RGD implies either partial unfolding of the enzyme or its proteolytic digestion. Here, we demonstrate that surface-absorbed thrombin promotes attachment and migration of endothelial cells through interaction with alpha(v)beta(3) and alpha(5)beta(1) integrins. Using site-directed mutants of thrombin we prove that this effect is mediated by the RGD sequence and does not require catalytic activity. The effect is abrogated when residues of the RGD sequence are mutated to Ala and is not observed with proteases like trypsin and tissue-type plasminogen activator, unless the RGD sequence is introduced at position 187-189. The potent inhibitor hirudin does not abrogate the effect, suggesting that thrombin functions through its RGD sequence in a non-canonical conformation. A 1.9-Angstroms resolution crystal structure of free thrombin grown in the presence of high salt (400 mm KCl) shows two molecules in the asymmetric unit, one of which assumes an unprecedented conformation with the autolysis loop shifted 20 Angstroms away from its canonical position, the 220-loop entirely disordered, and the RGD sequence exposed to the solvent.
Highlights
From the ‡Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St
Using site-directed mutants of thrombin we prove that this effect is mediated by the RGD sequence and does not require catalytic activity
The effect is abrogated when residues of the RGD sequence are mutated to Ala and is not observed with proteases like trypsin and tissue-type plasminogen activator, unless the RGD sequence is introduced at position 187–189
Summary
Previous studies have suggested that thrombin interacts with integrins in endothelial cells through its RGD (Arg-187, Gly-188, Asp-189) sequence. The anticoagulant role of the enzyme depends on the cleavage of protein C with the assistance of the cofactor thrombomodulin [4] In addition to these well characterized roles that require catalytic activity, thrombin promotes chemotaxis even when its active site is blocked [5]. Thrombin promotes endothelial cell adhesion upon interaction with glypican [6] by exploiting its RGD (Arg-187, Gly-188, Asp-189) sequence [7, 8], a motif involved in integrin binding [9] that is not present in many other proteases. The possible functional role of the RGD sequence in thrombin is called into question by the crystal structure where Arg187 is exposed to the solvent, but Gly-188 and Asp-189 are almost completely buried under the 220-loop [14]. The enzyme may expose the RGD sequence independent of denaturation or proteolysis, as reported here
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