Many proteins of eukaryotic cells undergo folding and modification in the lumen of the endoplasmic reticulum (ER). During these processes, misfolded proteins and unassembled protein complexes are first retrotranslocated into the cytosol and are eventually degraded by the proteasome. This multi-step process is called ER-associated degradation (ERAD). Recent studies revealed that many factors (
p97/VCP/Cdc48p, Ufd1, Npl4, E3 ubiquitin ligases, Derlin-1 and -2, VIMP and so on) are involved in ERAD.
p97 is one of the best characterized examples of AAA (ATPases associated with diverse cellular activities) family proteins.
p97 forms a complex with Npl4 and Ufd1 subunits, which are essential for the ERAD function of
p97. In concert with ubiquitination by ER-membrane resident E3 ubiquitin ligases,
p97 extracts ERAD substrates from the ER to the cytosol using energy produced by
p97-catalyzed ATP hydrolysis. Several reports have demonstrated that E3 ubiquitin ligases (Hrd1,
gp78 and Doa10p) directly associate with
p97. Extracted and ubiquitinated substrates will be transferred to the 26S proteasome. Although their biochemical properties have been well studied, their cellular functions in development have not been revealed. Here, we investigate cellular function of
p97 and E3 ubiquitin ligases in Caenorhabditis elegans as a model organism. We found that C. elegans possesses three ER resident E3 ubiquitin ligases (names as HRD-1, HRDL-1 and MARC-6) like mammals and that their simultaneous depletion caused extremely delayed growth. By monitoring the expression of an ER chaperone gene (
hsp-4::gfp), it was revealed that
p97 and HRD-1 play essential roles in unfolded protein response and ERAD pathways. We also found that HRD-1 functions in concert with BiP and that two BiP paralogues are functionally diversified. HRD-1 and BiP(HSP-3) play important roles in the developmental growth and intestinal functions, while HRD-1 and BiP(HSP-4) in the gonad formation. It has been reported that Doa10p functionally interacts with a cytosolic chaperone Ssa1p. It is thus reasonably assumed that E3 ubiquitin ligases function in concert with a specific partner chaperone. Elucidation of their interaction in the model organism will allow us to understand not only overall regulatory mechanism of protein quality control system but also human diseases associated with protein quality control system.