Figure 2

The ER & ERAD translocation system according to yeast. (a) In the co-translational pathway, SRP binds to the emerging polypeptide to form a RNC. Then, SRP is recognized by the SR composed of SRα and SRβ. The RNC is transferred to Sec61, which translocates the emerging protein into or across the ER membrane. (b) In the post-translational pathway, the precursor is guided by chaperones to the Sec62/63 and Sec61 complex. The final insertion or translocation of polypeptides is assisted by BiP, the ER-resident Hsp70 isoform [21]. (c) TA proteins are transferred to Get3 in a Sgt2/Get4/Get5 mediated manner. The Get3-TA protein complex interacts with Get1-Get2 complex, the latter facilitating the release of TA protein into the membrane. (d) In the ERAD-L pathway, misfolded substrates are recognized by ER-resident chaperones. This complex interacts with the luminal domain of Hrd3, the latter being complexed with the Hrd1/Der3 [22]. Their substrates are translocated via Sec61, Der1 or the E3 ligases. A complex of cytosolic E3-ligases (anchored to the membrane; Doa10, Hrd1/Der3) and E2-conjugating enzymes (Ubc6, Ubc7 bound to Cue1) ubiquitinate the substrates in the cytosol. Hrd1/Der3 interacts with Der1 via Usa1, while Dfm1 forms complexes with Doa10, Hrd1/Der3 and Cdc48. After ubiquitination, the substrates are pulled out by the AAA ATPase machinery (Cdc48, in complex with Npl4, Ufd1 and Ubx2) and degraded by the 26S proteasome. Note: The colour indicates the number of genes encoding the component. Green: equal number of genes found in Arabidopsis and tomato; blue: more genes in tomato than in A. thaliana; pink: less genes in tomato than in A. thaliana; yellow: no orthologues found in plants; red: only found in plants; purple: multiple subunits unified for better presentation; brown: not found in tomato; white: not included in our discussion.