Figure 2

Association between proteins (Streptavidin–biotin) anchored in the same bilayer (2-d in cis). \({I}_{1S}\) and \({I}_{2S}\) are the fluorescence intensities respectively of an \(S\) bound to a membrane-anchored \(B\) forming a \({B}_{1}S\) complex and a \({B}_{1}S\) to a second membrane-anchored \(B\) forming a \({B}_{2}S\) complex. (a) Results obtained for the variation of the ratio between the intensities \({I}_{2S}/{I}_{1S}\) for different initial molar ratios \({r}_{BS}={n}_{{B}_{0}}/{n}_{{S}_{0}}\) (where \({n}_{{B}_{0}}\) and \({n}_{{S}_{0}}\) are the initial quantities of \(B\) and \(S\) respectively). Results are compared over a wider range of \({r}_{BS}\) to numerical models from kinetic equations (see Eq. (S23) in Supplementary Information, Sect. 5) performed for \({\underline{k}}_{on,2.5d}\)= 10⁶ M^–1.s^–1 and \({\underline{k}}_{on,2d}\) = 2.5.10¹³ mol^–1.dm².s^–1 (- -) Error bars are not visible when they are lower than the size of the dot. (b) For small \({r}_{BS}\) values, \({I}_{2S}/{I}_{1S}\) varies linearly with \({r}_{BS}\) with a slope proportional to \({\underline{k}}_{on,2.5d}/{\underline{k}}_{on,2d}\) and a straight line adjustment (- -) gives \({\underline{k}}_{on,2.5d}/{\underline{k}}_{on,2d}\) = 6.4 ± 0.5 nm.