Past exam of the biochemistry course of the University of Cambridge / 2024 / Paper 5 / Section 2 / q / Solution

Some cell membrane delivery and fusion must continue in the biological cells whose buds actually grow, but it need not use normal long-range cable transport. secretory vesicles can move by molecular diffusion or other non-directed motion and be preferentially captured, tethered and fused where a surviving polarity cue concentrates the relevant vesicle docking machinery. Spatial selectivity can therefore reside in the destination rather than in directed travel over the entire route. Less efficient delivery is consistent with delayed budding and small buds.

Short residual actin tracks could also contribute; the experiment does not prove that every actin filament is absent. Neither bud shape nor Con-A staining directly measures vesicle trajectories or fusion events. Tracking secretory vesicles together with a membrane-fusion reporter and a polarity marker would distinguish directed runs from molecular diffusion followed by localized capture, with expression and imaging controls for reporter effects. Random arrival with selective vesicle docking is a published mechanistic proposal for this physiological context, rather than a demonstrated trajectory in the present images.^[ref]