Figure S1.
Motion and Force Generation of Phagosomes Inside Macrophage Cells: Calculation of the Area ACONTACT from Where Dyneins Can Contact an MT to Drive Cargo Transport, Related to Figures 1 and 3
(A) Representative video tracks of early phagosomes (EPs) and late phagosomes (LPs) moving inside J774 mouse macrophage cells. MT orientation is shown as a cartoon. Approximately linear (X-Y) trajectories were chosen, and the component of motion along a straight line (assumed to be a single MT) was calculated. EPs usually exhibit bidirectional motion, but LPs moved in unidirectional manner along the MT.
(B) Stall force records of EPs and LPs inside J774 cells are shown. The microtubule orientation (inferred from morphology of cells; see Rai et al., 2013) is also schematized along with an optical trap (red focused beam). EPs exhibit bidirectional stalls, but LPs show unidirectional stalls largely in the minus direction. This is broadly consistent with the stall records seen on EPs and LPs purified from Dictyostelium (main text).
(C) Left: A spherical cargo of radius R is shown in contact with a MT at the bottom of the cargo (contact point = B). Two dynein molecules (each of length D = 70nm) are attached to points P and P’ on the cargo, and at S and S’ on the MT. P’S’ = PS = D. The permissible arc along which cargo-bound dyneins can contact the MT is PP’ (shown in red). Dyneins attached to cargo beyond this arc are not long enough to reach the MT. The projection of arc PP’ on the MT is SS’, and is the maximum length along this MT on which cargo-bound dyneins can engage. The direction of dynein driven motion is shown (arrow).
Right: The same cargo is now visualized looking upward from beneath the MT (see blue arrow in left panel). Dyneins situated exactly atop the MT (along SS’) can engage the MT. Dyneins that are attached to cargo within a distance D perpendicular to the MT may also be able to reach the MT. Therefore, the maximum possible area of contact for dyneins ( = ACONTACT) can be approximated as the shaded rectangle (though this is likely an overestimate of the contact area).
Bottom (gray box) Arc length PP’ and ACONTACT are calculated in terms of R and D. If dyneins are randomly placed on the cargo, then the probability of dynein to attach within ACONTACT is the ratio of ACONTACT to total surface area of cargo ( = 4πR2). This probability is called PCONTACT, and is plotted as a function of R in Figure 3E (main manuscript).