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J. Biol. Chem., Vol. 281, Issue 35, 25466-25474, September 1, 2006

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Stepped Changes of Monovalent Ligand-binding Force during Ligand-induced Clustering of Integrin _IIB3^*

From the ^aInstitute of Microbiology and Immunology, National Yang-Ming University, Taipei, Taiwan, the ^bDepartment of Photonics & Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu, Taiwan, the ^cInstitute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan, the ^dDepartment of Physics, National Central University, Taoyuan, Taiwan, the ^eInstitute of Information Science, Academia Sinica, Taipei, Taiwan, the ^fDepartment of Electrical Engineering, Yuan-Ze University, Taoyuan, Taiwan, the ^gDepartment of Electrophysics, National Chiao-Tung University, Hsinchu, Taiwan, the ^hInstitute of Biophotonic Engineering, National Yang-Ming University, Taipei, Taiwan, the ⁱDepartment of Surgery, Veteran General Hospital, Taipei, Taiwan, and the ^jDivision of Medical Research, Taipei City Hospital, Taipei, Taiwan

Recent evidence demonstrated that conformational changes of the integrin during receptor activation affected its binding to extracellular matrix; however, experimental assessment of ligand-receptor binding following the initial molecular interaction has rarely been carried out at a single-molecule resolution. In the present study, laser tweezers were used to measure the binding force exerted by a live Chinese hamster ovary cell that expressed integrin _IIb3 (CHO _IIb3), to the bead carrier coated with the snake venom rhodostomin that served as an activated ligand for integrin _IIb3. A progressive increase of total binding force over time was noticed when the bead interacted with the CHO _IIb3 cell; such an increase was due mainly to the recruitment of more integrin molecules to the bead-cell interface. When the binding strength exerted by a single ligand-receptor pair was derived from the "polyvalent" measurements, surprisingly, a stepped decrease of the "monovalent binding force" was noted (from 4.15 to 2.54 piconewtons (pN)); such decrease appeared to occur during the ligand-induced integrin clustering process. On the other hand, the mutant rhodostomin defective in clustering integrins exhibited only one (1.81 pN) unit binding strength.

Received for publication, February 24, 2006 , and in revised form, June 13, 2006.

^* This work is supported by grants from National Science Council (NSC 94-2627-B-010-004), a grant from Ministry of Education, Aim for the Top University Plan, and Center for Nano-Science and Technology/University System of Taiwan (awarded to C.-H. L.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ These two authors contributed equally to this paper.

² To whom correspondence should be addressed: Institute of Microbiology and Immunology, Institute of Biophotonics Engineering, National Yang-Ming University, Taipei, Taiwan. Tel.: 886-2-28267219; Fax: 886-2-28212880; E-mail: linch{at}ym.edu.tw.

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