Final answer:
Dynein is the molecular motor protein used for retrograde axonal transport.
Step-by-step explanation:
The molecular motor protein used for retrograde axonal transport is Dynein. Dynein is an ATP-dependent motor protein that powers the movement of cargo in the opposite direction of kinesin. In the context of retrograde axonal transport, dynein is responsible for moving empty vesicles back to the cell body along microtubule tracks in the axons.