Practice question

Tail-anchored proteins pose topological challenge because single transmembrane domain near extreme C-terminus, often within last thirty residues, emerges from ribosome only after termination codon, precluding co-translational SRP recognition. Cellular solution is Guided Entry of Tail-anchored proteins, GET pathway. Newly released tail-anchored client is captured in cytosol by pre-targeting complex Sgt2-Get4-Get5 that hands hydrophobic tail to dimeric ATPase Get3 in yeast, TRC40 in mammals, forming closed dimer that shields tail via methionine-rich groove similar to SRP54. ATP binding closes dimer protecting tail from aqueous environment preventing aggregation. Complex docks to ER membrane receptors Get1-Get2, mammalian orthologs WRB and CAML, multi-pass membrane proteins that insert tail into bilayer using energy from ATP hydrolysis triggering Get3 opening and release. This pathway handles many SNAREs such as synaptobrevin, syntaxin-5, and apoptosis regulators Bcl-2. SRP recognizes N-terminal signals during translation, Ran controls nuclear import, BiP works lumenally as ratchet, so Get3/TRC40 defines distinct post-translational pathway dedicated to C-terminal membrane insertion and organelle targeting specificity for essential cellular functions. Additional coordination with cellular stress pathways ensures fidelity, prevents aggregation, and links trafficking to growth control and proteostasis maintenance across diverse cell types and developmental stages.