The physics of chaperones

A simple physical property based on entropy gives Hsp70 a pulling force, say Paolo De Los Rios (EPFL, Lausanne, Switzerland), Pierre Goloubinoff (University of Lausanne, Switzerland), and colleagues. The new theory resolves two deadlocked models.

Hsp70s have two major activities: they help unfold stable protein aggregates, and they pull proteins through membrane channels. In the Brownian ratchet model for protein import, Hsp70 grabs part of a protein once it is spontaneously unfolded by random thermal fluctuations, and passively prevents it from sliding backward. By contrast, the power stroke model has the Hsp70 using a channel protein as the fulcrum for a lever arm movement that yanks the protein inwards.

The Lausanne group suggested that an emerging protein with a bulky Hsp70 attached would keep bumping against the nearby membrane. But if the whole...