Hsp70 cannot bind runs of glutamine acid (left) or glycine (right ).

Neupert/EMBO

Amotor protein, by its very name, should really be pulling on something or exerting force. But Koji Okamoto, Walter Neupert (Universität München, München, Germany), and colleagues claim that the mitochondrial import motor works not by active pulling but by using a Brownian ratchet.The motor in question is heat shock protein 70 (Hsp70) in the mitochondrial matrix. ATP-bound Hsp70 is bound to the import channel and the incoming protein. Some researchers have suggested that ATP hydrolysis drives an Hsp70 power-stroke that drags a protein into the mitochondrial matrix. But Neupert and colleagues show that the system can import a protein with titin immunoglobulin domains, which have been shown to require >200 pN of force to unfold. In general, molecular motors can only generate ∼5 pN of force, suggesting that Hsp70 cannot be using...

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