STT3B gets the tip

To glycosylate the C-terminal tail of a protein, cells turn to a little understood enzyme subunit, Shrimal et al. reveal.

Affixing oligosaccharides to asparagine residues in a protein, a process known as N-linked glycosylation, can change how that protein folds and works and whether it gets secreted. Some proteins require these added groups at their C terminus. The protein complex responsible for glycosylation is oligosaccharyltransferase (OST). The OST subunit that catalyzes the modification is STT3, which comes in two isoforms, STT3A and STT3B. However, STT3A might skip sites at the C terminus of a protein.

Shrimal et al. found that STT3B picks up the slack. Sex hormone–binding globulin, for example, has two glycosylation sites near its C terminus. The researchers showed that siRNA that targeted STT3A had no effect on glycosylation of the protein, whereas siRNA targeting STT3B often left the C-terminal sites bare.

By adding amino acids to transferrin, which harbors two glycosylation sites, the team determined that STT3A can glycosylate acceptor sites that are more than 50 to 55 amino acids from the end of the protein. That makes sense, given its location. OST is embedded in the ER membrane, and STT3A glycosylates a growing protein strand as it feeds into the ER. However, STT3A can’t modify the C-terminal tip before translation finishes and the new protein is released from the ribosome. That’s when STT3B steps in and glycosylates the remaining sites on the completed protein, although how STT3B finds its targets in the ER remains a mystery.