Your challenge today is to invent a machine that can push a wet noodle through a straw. It can’t pull it. First it has to grab the end, then push it through without breaking it. Oh, and there’s a catch; the straw has a plug at the far end and a constriction inside. Give up? Maybe you should watch how cells do it. The mechanism was described by Anastassios Economou in Nature this week.1
Cells have to do this kind of thing all the time, so they have specialized machinery for the task. The wet noodles are protein chains in their unfolded state. The straws are narrow channels through membranes that are normally in a plugged configuration. Just outside the straw entrance are several precisely-fitted proteins that first attract the chain and cradle it gently between two halves that swivel shut. As one half tilts, it causes the constriction in the tunnel to open up. The two parts then fit together like hands, and use a powered motor to gently send the noodle through.
Economou included a schematic diagram of the five-part mechanism that pushes the proteins through. (He included a couple of stylized hands to show how the delicate grasping and pushing is done.) Here’s the caption:
This simplified representation is based on both earlier studies and the new findings. In this cut-away view of the membrane, the SecA motor lies flat against the cytoplasmic side of the SecY channel (yellow), and consists of a two-domain ATP-powered engine (light and dark blue) and two ‘business-end’ domains (green and magenta; depicted as hands). a, Initially, the channel pore is sealed by both a constriction halfway through it and a mobile plug domain (not shown) near its exit. The pre-protein-binding domain of the motor (magenta) is in the open state, exposing an elongated corridor that connects to the entrance of the channel. This open state is seen in structures of the isolated motor. b, Swivelling this domain around its stem would allow it to embrace a secretory protein chain. At this stage, a finger (green) from the second hand of SecA might be in close contact with the chain. c, When ATP (not shown) is present, the engine conformation changes and the finger could move upwards, pushing or dragging the protein chain into the pore. This motion, or other conformational changes, leads to the opening of the pore….
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