The disintegrin echistatin stabilizes integrin alphaIIbbeta3's open conformation and promotes its oligomerization.
We have employed echistatin, a 5.4 kDa snake venom disintegrin, as a model protein to investigate the paradox that small ligand-mimetics can bind to the resting alphaIIbbeta3 integrin while adhesive macromolecules cannot. We characterized the interactions between purified human alphaIIbbeta3 and two recombinant echistatin variants: rEch (1-49) M28L, chosen for its selectivity toward beta3-integrins, and rEch (1-40) M28L, a carboxy-terminal truncation mutant. While both contain an RGD integrin targeting sequence, only rEch (1-49) M28L was an effective inhibitor of alphaIIbbeta3 function. Electron microscopy of rotary shadowed specimens yielded a variety of alphaIIbbeta3 conformers ranging from compact, spherical particles (maximum dimension 22 nm) to the classical "head with two tails" forms (32 nm). The population of larger particles (42-56 nm) increased from 17% to 28% in the presence of rEch (1-49) M28L, indicative of ligand-induced oligomerization. Sedimentation velocity measurements demonstrated that both full length and truncated echistatin perturbed alphaIIbbeta3's solution structure, yielding slower-sedimenting open conformers. Dynamic light scattering showed that rEch (1-49) M28L protected alphaIIbbeta3 from thermal aggregation, raising its transition mid-point from 46 degrees C to 69 degrees C; a smaller shift resulted with rEch (1-40) M28L. Sedimentation equilibrium demonstrated that both echistatin ligands induced substantial alphaIIbbeta3 dimerization. van't Hoff analysis revealed a pattern of entropy/enthalpy compensation similar to tirofiban, a small RGD ligand-mimetic that binds tightly to alphaIIbbeta3, but yields smaller conformational perturbations than echistatin. We propose that echistatin may serve as a paradigm for understanding multidomain adhesive macromolecules because its ability to modulate alphaIIbbeta3's structure resides on an RGD loop, while full disintegrin activity requires an auxiliary site that includes the carboxy-terminal nine amino acid residues.
Hantgan
Roy R
RR
Department of Biochemistry, Wake Forest University School of Medicine, Winston-Salem, NC 27517, USA. rhantgan@wfubmc.edu
Stahle
Mary C
MC
Connor
John H
JH
Lyles
Douglas S
DS
Horita
David A
DA
Rocco
Mattia
M
Nagaswami
Chandrasekaran
C
Weisel
John W
JW
McLane
Mary Ann
MA
D000818:Animals; D001792:Blood Platelets / Q000378:metabolism; D019281:Dimerization; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins; D008024:Ligands; D008854:Microscopy, Electron; D008958:Models, Molecular; D009154:Mutation; D009842:Oligopeptides / Q000737:chemistry / Q000494:pharmacology*; D010455:Peptides / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D010974:Platelet Aggregation; D010975:Platelet Aggregation Inhibitors / Q000737:chemistry / Q000378:metabolism*; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000737:chemistry* / Q000378:metabolism; D011487:Protein Conformation / Q000187:drug effects*; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism*; D014757:Viper Venoms / Q000737:chemistry
D036341:Intercellular Signaling Peptides and Proteins; D008024:Ligands; D009842:Oligopeptides; D010455:Peptides; D010975:Platelet Aggregation Inhibitors; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D011994:Recombinant Proteins; D014757:Viper Venoms; C057986:echistatin