ORIGINAL PAPER Surface-bound selectin–ligand binding is regulated by carrier diffusion Ganyun Sun Æ Yan Zhang Æ Bo Huo Æ Mian Long Received: 23 October 2008 / Revised: 6 February 2009 / Accepted: 15 February 2009 / Published online: 10 March 2009 Ó European Biophysical Societies’ Association 2009 Abstract Two-dimensional (2D) kinetics of receptor– ligand interactions governs cell adhesion in many biological processes. While the dissociation kinetics of receptor– ligand bond is extensively investigated, the association kinetics has much less been quantified. Recently receptor– ligand interactions between two surfaces were investigated using a thermal fluctuation assay upon biomembrane force probe technique (Chen et al. in Biophys J 94:694–701, 2008). The regulating factors on association kinetics, however, are not well characterized. Here we developed an alternative thermal fluctuation assay using optical trap technique, which enables to visualize consecutive binding– unbinding transition and to quantify the impact of micro- bead diffusion on receptor–ligand binding. Three selectin constructs (sLs, sPs, and PLE) and their ligand P-selectin glycoprotein ligand 1 were used to conduct the measure- ments. It was indicated that bond formation was reduced by enhancing the diffusivity of selectin-coupled carrier, sug- gesting that carrier diffusion is crucial to determine receptor–ligand binding. It was also found that 2D forward rate predicted upon first-order kinetics was in the order of sPs [ sLs [ PLE and bond formation was history-depen- dent. These results further the understandings in regulating association kinetics of surface-bound receptor–ligand interactions. Keywords Association kinetics  Regulating factors  Carrier diffusion  Optical trap  Selectin  Ligand Introduction Cell adhesion mediated by receptor–ligand interactions is crucial to such biological processes as inflammatory reac- tion (Springer 1995), tumor metastasis (Albelda 1993), arteriosclerosis (Galkina and Ley 2007), and wound heal- ing (Grinnell 1992). To mediate cell adhesions, receptors and their ligands must be anchored onto two apposed surfaces, which is so-called two-dimensional (2D) inter- action. This is different from three-dimensional (3D) interaction where at least one of receptors and ligands is in fluid phase (Bell 1978). 2D association and dissociation kinetics governs the formation and rupture of surface- bound receptor–ligand bond. Direct measurement of 2D kinetics rates and governing factors is indispensable to understand the biophysical bases of receptor–ligand inter- actions in regulating cell adhesions. 2D dissociation kinetics (Hammer and Lauffenburger 1987; Alon et al. 1995; Chen et al. 1997) and forced bond rupture (Florin et al. 1994; Dammer et al. 1996; Tees et al. 2001) as well as their regulating factors (Evans et al. 2001; Levin et al. 2001; Huang et al. 2004; Marshall et al. 2005; Wu et al. 2007) have been investigated theoretically and experimentally using various approaches or assays, e.g., flow chamber (Kaplanski et al. 1993; Alon et al. 1995; Finger et al. 1996; Yago et al. 2007; Paschall et al. 2008), biomembrane force probe (BFP) (Evans et al. 2001; Evans and Ritchie 1997), atom force microscopy (AFM) (Fritz et al. 1998; Merkel et al. 1999; Marshall et al. 2005; Lu ¨ et al. 2006), micropipette aspiration (Chesla et al. 1998; Long et al. 2001; Shao and Xu 2002), optical tweezers (Kulin et al. 2002; Rinko et al. 2004), fluorescence recovery after photo- bleaching (FRAP) (Dustin et al. 1996; Tolentino et al. 2008). Only a few works, however, were focused on quan- tifying 2D association kinetics mainly due to theoretical and G. Sun  Y. Zhang  B. Huo  M. Long (&) National Microgravity Laboratory and Center for Biomechanics and Bioengineering, Institute of Mechanics, Chinese Academy of Sciences, 100190 Beijing, People’s Republic of China e-mail: mlong@imech.ac.cn 123 Eur Biophys J (2009) 38:701–711 DOI 10.1007/s00249-009-0428-y