The early steps of ER protein targeting
• How does a cell prevent aggregation of membrane proteins during delivery to the ER?
• How is the binding of targeting factors with the ribosome regulated in time and space?
DAS IST EIN ABSATZ
Cotranslational targeting by SRP prevents aggregation of membrane proteins
The initial steps of the SRP pathway. Signal recognition particle (SRP) is a ribonucleoprotein complex, which interacts with translating ribosomes and binds to nascent polypeptide chains, containing an SRP recognition sequence. Transmembrane (TM) domains of ER-targeted proteins, which serve as SRP recognition sequences are termed signal anchor sequence (SA).
(1) The ribosome synthesizes a polypeptide containing a TM domain. (2) When translation continues, the TM domain emerges from the ribosomal tunnel cotranslationally and serves as a SA sequences. (3) SRP binds to the exposed SA sequence and targets the ribosome nascent chain complex to the SRP receptor in the ER membrane.
Posttranslational targeting of membrane proteins involves release to the cytosol.
The initial steps of the GET pathway. ER-targeted proteins which contain only a single transmembrane domain at their very C-terminus are termed tail-anchored (TA) proteins. TA proteins reach the ER via the posttranslational guided entry of TA protein (GET) pathway, which involves the dynamically interacting protein components Sgt2, Get4/5, Get3, and the membrane insertase Get1/2.
(1) The ribosome synthesizes a TA protein. (2) After ribosome-release the TA protein is captured by the chaperone-like protein Sgt2, which shields the hydrophobic TA sequence and prevents aggregation. (3) The Sgt2/TA protein complex then recruits the Get4/5 complex, which interacts with Sgt2 via Get5. (4) As a next step Get3 binds to Get4 and forms the pre-targeting complex. (5) Within this complex the TA protein is transferred to Get3. (6) Get3 bound to the TA protein dissociates and delivers the TA protein to the insertase in the ER membrane.
Latest lab news on the GET pathway:
Our recent findings reveal that aggregation of TA proteins in the cytosol is minimized by the recruitment of Sgt2 to the ribosomal tunnel exit. This is achieved by Get4/5, which binds to the ribosome in close proximity of the tunnel exit and recruits Sgt2 (below cartoons; read more, also about the competition between the SRP and GET pathways, in: Zhang et al. 2021.