Derlins were originally proposed to be transmembrane channels though which ERAD substrates are dislocated across the ER membrane1,2
. However, the structure of the homologous rhomboid protease GlpG, the only member of this protein family for which a high resolution structure exists, lacks any evident path through the membrane and is thus inconsistent with a channel-like function. Therefore, barring major structural differences between Derlins and GlpG, or induced conformational changes, it is unlikely that Derlins themselves function as dislocation channels.
The destabilization and unfolding of polypeptide substrates is a prerequisite to their hydrolysis by rhomboid proteases, as scissile peptide bonds within the membrane are normally hidden within transmembrane α-helices. The Derlin-1 rhomboid domain may, together with p97/VCP, promote unfolding of the integral membrane oligomeric complexes formed by the dislocation machinery48
, thereby facilitating disassembly of the dislocation machinery and enabling the subsequent release of substrates following their transfer across the membrane. Complete loss of Derlin function would lead to a failure to disassemble ERAD complexes for a second round of dislocation, and this may explain why pre-incubation of microsomes with antibodies against Derlin-1 was observed to block the in vitro
dislocation of mutant pro-α-factor only after a short lag in which the initial rate of dislocation was unaltered14
Finally, Derlins may interact directly with substrates to enable their extraction from the ER membrane. Derlins and p97/VCP may function in an analogous fashion to the mitochondrial rhomboid Pcp1, which also cooperates closely with a AAA ATPase, m
-AAA, to dislocate and cleave Ccp1 within the mitochondrial inner membrane49
. The irregular shape of rhomboids, due in part to the amphipathic L1 loop and their small hydrophobic thickness, has been proposed to substantially compress and deform the bilayer to facilitate the unfolding and exposure of substrate cleavage sites33
. This property may endow Derlins with the ability to facilitate the release of substrates from a membrane-associated state during dislocation.
Of the thirteen predicted rhomboid genes in the human genome, including Derlins, only five encode the conserved catalytic Ser-His dyad (Supplementary Fig. 2
). The biological functions of inactive rhomboids are largely unknown. However, one recent report50
demonstrates that the ER-localized inactive rhomboid iRhom functions as a negative regulator of neuronal EGFR signaling in Drosophila melanogaster
by facilitating the degradation of EGFR ligands by the proteasome, thereby establishing a functional link between a rhomboid pseudoprotease and ERAD. We propose that Derlins represent an expansion of this function in the broader context of ER quality control.