Geometry of the γ HC–LC1–microtubule ternary complex in situ. (a) The principal molecular axes of LC1 are superimposed on the ensemble of NMR-derived backbone structures (Protein Data Bank accession no. 1M9L) and the total axial dimensions indicated. LC1 is highly asymmetric with an axial ratio of 1.00 (green):1.13 (yellow):2.46 (red). (b) Thin section electron micrograph of the outer dynein arm in situ. The regions occupied by the motor domains of the three HCs are indicated as are the approximate distances between the γ HC motor unit and the neighboring outer doublet microtubules. (c) Three-dimensional reconstruction of the outer dynein arm–microtubule rigor complex from cryo-EM tomograms. Pink, α HC; orange, β HC; yellow, γ HC. There is a clear connection (arrow) between the γ HC motor unit and the microtubule to which the dynein is bound. The reconstruction is modified from Oda et al. (2007). (d) Models illustrating two potential geometries for the γ HC–LC1–microtubule complex in which LC1 is proposed to tether the γ HC to the A-tubule of the doublet to which dynein is permanently attached via the IC–LC complex and the docking complex. The HC color scheme is the same as in c. These models are based on the known dimensions of LC1, the measured motor unit/microtubule distances, and the immunogold localization of LC1 to the A-tubule. (left) The γ HC motor unit is in the same orientation as those of the α and β HCs. In this situation, it is unlikely that both copies of LC1 could interact simultaneously with the A-tubule. Thus, it is possible that the copy of LC1 that is bound switches as the AAA motor unit alters conformation during the mechanochemical cycle. (right) The γ HC motor unit is oriented differently such that both copies of LC1 bind the A-tubule simultaneously. In this situation, the system may act as a brake to limit sliding rather than as a microtubule translocase.