The distribution of kinesin-1 fits with a Loose Bucket Brigade.

<p>(A–D) Models for kinesin-1 transport and recycling in neuronal processes. In the Diligent Worker model (A), active kinesin-1 motors (green) transport vesicular cargoes to the plus ends of the microtubules (solid green arrow). The motors disengage from the cargo and track (black arrow), assume the inactive autoinhibited conformation (red), and diffuse in the neurite (dotted red arrows). In the Loose Bucket Brigade (B), active kinesin-1 motors (green) transport vesicular cargoes to the plus ends of the microtubules (solid green arrow) but stochastically fall off the cargo (orange arrows). These motors diffuse within the neurite (dotted red arrows) and can reattach to cargoes (green arrows) and again contribute to the transport of vesicles to the plus ends of microtubules. The Hybrid 1 model (C) assumes that kinesin-1 motors can stochastically fall off the cargo (orange arrows) but cannot reattach while diffusing in the neurite (like the Sloppy Bucket Brigade during active transport and the Diligent Worker during recycling). The Hybrid 2 model (D) assumes that kinesin-1 motors do not fall off during transport but can reattach to cargoes while diffusing in the neurite (like the Diligent Worker during active transport and the Sloppy Bucket Brigade during recycling). For each model, computer simulations were used to determine the predicted distribution of kinesin-1 motors in neuronal processes (bottom panels). (E, F) Experimental determination of kinesin-1 distribution in neuronal processes. (E) Differentiated CAD cells or (F) primary hippocampal neurons expressing EGFP (a marker of cell volume) were stained with an antibody to KHC. The fluorescence intensity of KHC (red lines) and EGFP (green lines) was plotted along the length of the neurite or axon (right panels). The fluorescence intensity of KHC represents the spatial distribution of the total kinesin concentration (a sum of concentrations of bound and freely diffusing kinesin-1). (G) The increasing portion of the kinesin-1 profile (red line) in the neuronal process of the CAD cell in (E) was plotted point-by-point as a ratio of the local fluorescence intensities of KHC and EGFP. The distribution fits with an exponential curve (blue line) and is consistent with the Loose Bucket Brigade model.