Ribosome-bound Get4/5 facilitate the capture of tail anchored proteins by Sgt2 in yeast

The guided entry of tail-anchored proteins (GET) pathway assists in the proper delivery of tail-anchored (TA) proteins to the ER. Here we uncover how the yeast GET pathway components Get4/5 mediate capture of TA proteins by Sgt2, which interacts with TA sequences and hands them over to the targeting component Get3. Get4/5 binds directly and with high affinity to ribosomes, positions Sgt2 close to the ribosomal tunnel exit, and facilitates the capture of TA proteins by Sgt2. The contact sites of Get4/5 on the ribosome overlap with those of SRP, the factor mediating cotranslational ER-targeting of proteins containing internal TM domains. Exposure of nascent, internal TM domains at the tunnel exit induces high-affinity ribosome-binding of SRP, which in turn prevents ribosome-binding of Get4/5. In this way, the position of TM domains within nascent ER-targeted proteins mediates partitioning into either the GET or SRP pathway directly at the ribosomal tunnel exit. Wild-type BY4741 yeast cells or cells expressing genomically C-terminally His-Tev-ProtA-tagged Get4 were analysed by CRAC (Bohnsack et al., 2012).