Featured Article - October 2009
Short description: Unlike most membrane proteins, SNAREs enter the endoplasmic reticulum after translation. The structure of the ATPase needed for entry reveals the likely mechanism.
In eukaryotic cells, membrane proteins are sent to the endoplasmic reticulum (ER) during or shortly after synthesis. Most membrane proteins possess a signal recognition peptide that ensures that the new protein is co-translationally targeted to the ER. However, a small but very important group that includes the SNARE proteins needed for vesicle trafficking and the Bcl2 family members needed for apoptosis use a post-translational pathway.
This group, called the tail-anchored proteins, has a single C-terminal transmembrane domain (TMD) that holds the targeting information. In yeast, this domain interacts with a highly conserved ATPase called Get3 (Asna1 or TRC40 in mammals) to form a complex that is then targeted to the ER. Mateja et al. have now solved the X-ray crystal structure of Get3 in both nucleotide-free and nucleotide-bound conformations, revealing the likely mechanism for Get3 binding.
Both structures are symmetrical homodimers comprising ATPase and α-helical subdomains. In the nucleotide-free or 'open' state the α-helical subdomains have a space between them of at least 20 Å. The charged surface of this gap suggests that it may have a function other than binding substrates' TMDs. The nucleotide-bound state, however, undergoes a conformational change that brings the α-helical domains together, creating a solvent-exposed hydrophobic groove that spans both monomers.
Extensive mutagenesis of the residues within this groove showed that these hydrophobic amino acids are needed for substrate binding. In addition, the structure indicated that the nucleotide-binding sites are located near the ATPase switch II region, linking nucleotide binding to formation of the TMD-binding site.
A. Mateja et al. The structural basis of tail-anchored membrane protein recognition by Get3.
Nature (2009). doi:10.1038/nature08319