SIGNIFICANCE OF RGD LOOP AND C-TERMINAL DOMAIN OF ECHISTATIN FOR RECOGNITION OF ALPHA-IIB-BETA-3 AND ALPHA-V-BETA-3 INTEGRINS AND EXPRESSION OF LIGAND-INDUCED BINDING-SITE
C. Marcinkiewicz et al., SIGNIFICANCE OF RGD LOOP AND C-TERMINAL DOMAIN OF ECHISTATIN FOR RECOGNITION OF ALPHA-IIB-BETA-3 AND ALPHA-V-BETA-3 INTEGRINS AND EXPRESSION OF LIGAND-INDUCED BINDING-SITE, Blood, 90(4), 1997, pp. 1565-1575
Echistatin is a viper venom disintegrin containing RGD loop maintained
by disulfide bridges. It binds with a high affinity to alpha v beta 3
and alpha IIb beta 3 and it induces extensive conformational changes
in these integrins resulting in expression of ligand-induced binding s
ite (LIES) epitopes, We investigated the activities of echistatin and
its three analogues (R24A, D27W, echistatin 1-41). R24A echistatin did
not react with alpha IIb beta 3 and alpha v beta 3 integrins and did
not cause LIES effect, D27W echistatin showed increased binding to alp
ha IIb beta 3 and decreased binding to alpha v beta 3. This substituti
on impaired the ability of echistatin to induce LIES in alpha v beta 3
integrin. Deletion of nine C-terminal amino acids of echistatin decre
ased its ability to bind alpha IIb beta 3 and inhibit platelet aggrega
tion. Truncated echistatin failed to induce LIES epitopes on cells tra
nsfected with alpha IIb beta 3 and alpha v beta 3 genes. The ability o
f echistatin 1-41 to compete with binding of vitronectin to immobilize
d alpha v beta 3 and monoclonal antibody 7E3 to platelets and to VNRC3
cells was decreased, although this analogue, after immobilization, re
tained its ability to bind purified alpha v beta 3. We propose a hypot
hesis in which echistatin's RGD loop determines selective recognition
of alpha IIb beta 3 and alpha v beta 3 integrin, whereas the C-termina
l domain supports its binding to resting integrin and significantly co
ntributes to the expression of LIES epitope and to conformational chan
ges of the receptor, leading to a further increase of the binding affi
nity of echistatin and of the inhibitory effect. (C) 1997 by The Ameri
can Society of Hematology.