Trueman et al. describe how the Sec61 channel decides whether to open up and let proteins pass through it into the endoplasmic reticulum (ER).
Proteins are directed to the ER by hydrophobic signal sequences and by cytosolic accessory factors that deliver them—either co- or posttranslationally—to Sec61 channels in the ER membrane. To permit protein translocation, Sec61’s α subunit undergoes a conformational change that unblocks its central pore and opens up a “lateral gate” through which signal sequences can be inserted into the channel. To understand how this conformational change is triggered, Trueman et al. focused on a cluster of polar residues on either side of the lateral gate and on an adjacent patch of apolar residues in Sec61α’s “plug domain,” which blocks the central pore when the channel is closed.
Replacing these residues with polar amino acids boosted Sec61’s activity in budding yeast, enabling the channel to translocate proteins with minimally hydrophobic signal sequences that would normally remain in the cytosol. But substituting hydrophobic amino acids for the lateral gate polar residues inhibited Sec61’s activity. Many proteins were no longer transported into the ER, although proteins with highly hydrophobic signal sequences, such as the transmembrane domains of integral membrane proteins, were still translocated normally.
The lateral gate and plug domain residues therefore represent a “gating motif” that opens up the Sec61 channel to proteins with sufficiently hydrophobic signal sequences. Mutations in the motif—which is conserved in bacteria—affect channel function by altering the stability of its closed conformation. Senior author Reid Gilmore now wants to investigate how the cytosolic factors that assist protein translocation influence channel opening.
Text by Ben Short