The inner membrane of a chloroplast bristles with enzymes and other proteins that are synthesized in the cytoplasm, but how they get into position is uncertain. On page 249, Li and Schnell report that at least some proteins pass through the inner membrane and reenter it from the back side, rather than stopping on the way through.

Chloroplasts are swaddled by inner and outer membranes and contain a third membrane system, the thylakoid, where photosynthesis occurs. Most of the proteins in these membranes are made in the cytoplasm. Proteins can slip directly into the outer membrane. But the journey to the inner membrane is more complicated because proteins must traverse the outer membrane and avoid piercing the thylakoid layer.

Li and Schnell followed the path of atTic40, which is part of an inner membrane protein complex that helps usher others through the barrier. They found that chloroplasts import an elongated version of atTic40 that accumulates fully inside the chloroplast before moving back into the inner membrane.

The scientists also demonstrated that atTic40 can integrate into isolated inner membranes, indicating that import of the proteins and their insertion into the inner membrane are independent. Chloroplasts have thus evolved a sorting system that can distinguish inner membrane and thylakoid proteins.