SNDing proteins into the membrane

Recent structural study reveals a new route for membrane protein insertion in fungi

To the point

• Membrane proteins: A significant proportion of all cellular proteins are membrane proteins, essential for functions like transport and signaling.
• Translocon complex: The SND3 protein plays a crucial role in inserting proteins into the ER membrane, as part of a larger translocon complex associated with ribosomes.
• Analysis of the translocon: Researchers successfully characterized the SND3 translocon using electron cryomicroscopy.
• Evolution of protein folding: The SND3 translocon complex provides a missing puzzle piece in our understanding of how membrane protein folding evolved.

Around a third of all proteins in a cell sit within membranes, where they carry out fundamental tasks like transporting molecules, sending signals or transferring energy. However, these proteins are first produced in the cell’s cytoplasm by ribosomes and must be correctly incorporated into membranes, most often into the endoplasmic reticulum (ER) membrane. Due to their diverse structures and functions, membrane proteins are delivered into the ER membrane by different specialised pathways and machines. An important route is the so-called ‘SND-pathway’, but so far, the details of how it works remain mysterious.

Now, the Membrane Protein Biogenesis group, led by Melanie McDowell, has discovered that the SND3 protein is responsible for inserting proteins into the membrane as part of a larger ‘SND3 translocon’ complex bound to the ribosome. As a key part of the study, postdoctoral researcher Tzu-Jing Yang resolved an electron cryomicroscopy structure of the SND3 translocon. This allowed a detailed characterisation of the complex. However, isolating and analysing the structure of membrane proteins can be a challenge. Part of the success of the McDowell group is due to their expertise with the fungus Chaetomium thermophilum. This fungus naturally grows at high temperatures, producing more stable proteins that are ideal for structural studies.

From the ribosome into the membrane

The structure revealed that the SND3 translocon sits just beneath the tunnel where new proteins exit the ribosome. This position allows the complex to insert proteins directly into the ER membrane as they are produced. The SND3 translocon itself consists of several components that span the membrane. One of these is the SEC translocon, a well-known central hub for protein import into the ER. In the new structure, the authors found that the SEC translocon is blocked and closed, meaning that membrane proteins must take an alternative route. Working together with the department of Theoretical Biophysics, led by Gerhard Hummer, they showed that the SND3 protein has the characteristics of a previously unknown membrane insertase. This finding suggests the SND3 protein itself is the component within the SND3 translocon responsible for inserting proteins into the ER membrane, rather than SEC, as originally expected.

The newly discovered SND3 translocon mechanism is similar to a protein insertion mechanism recently found in a ‘multipass translocon’ in animals. As its name suggests, the multipass translocon inserts proteins that cross the membrane multiple times, implying that the SND3 translocon may have a similar preference for this type of protein. Interestingly, the SND3 component can only be found in certain eukaryotic organisms, suggesting that they have evolved unique ways to insert membrane proteins. Some examples include pathogenic fungi such as Candida albicans and Aspergillus fumigatus, along with the Trypanosoma parasites that cause sleeping sickness in humans.

Understanding the 3D structure of the SND3 translocon is crucial for learning about the evolution of protein insertion mechanisms and could open the door to therapeutic treatments targeting these membrane machines in the future.