Function And Mechanism Of SRP14 In ER Stress

In eukaryotes,most secretory proteins and membrane proteins are targeted to the endoplasmic reticulum(ER)for folding and maturation.However,the load of unfolded and misfolded proteins in the ER may overwhelm the capacity of the cellular machinery that copes with load because of numerous types of internal or external insults.The cell state in which misfolded or unfolded proteins are aberrantly accumulated in the ER is named ER stress.ER stress induces an evolutionarily conserved adaptive response,named the unfolded protein response(UPR),that deploys a piece of self-regulated machinery to maintain cellular proteostasis.From the late 1980s to early 2000,the three UPR sensors are identified mainly from the genetic screens and their downstream pathways are reported later.The three core branches of UPR are PRK-like ER kinase(PERK)-eukaryotic translation initiation factor2?(eIF2?),inositol-requiring protein 1?(IRE1?)-X-box binding protein 1(XBP1)and activating transcription factor 6?(ATF6).They alleviate the ER stress by slowing down the rate of protein translation,increasing protein folding capability,and promoting protein degradation.If the load cannot be relieved,it can induce the cells to apoptosis.It is reasonable that decreased protein transport rate can be a protective measure to relieve the burden of unfolded protein in the ER.Pre-emptive quality control and Co-translational degradation describe the phenomenon from a similar perspective.ER proteins are selectively rerouted to the cytosol for degradation under ER stress.However,studies that support this mechanism are still rare up to now.Moreover,the most important part of how cells decrease the ER proteins transport during ER stress has not been explained.Consequently,translocational attenuation is still an unappreciated protective response to ER stress.In this study,we first constructed a LoVo-SP-NHK-GFP system that could monitor the cell ER stress state in a real-time manner.Then,an inducible knockdown system was introduced to knock down HRD1,which could accumulate proteins in the ER including NHK,and ER stress was induced.The proteome,transcriptome,and exome were used to analyze the stressed samples from the HRD1 knockdown cells and found the protein expression of SRP14,the key component of SRP,was significantly decreased.Furthermore,the down-regulation of SRP14 under ER stress existed in several cell lines and tissues and was dependent on the ubiquitin-proteasome system.SRP14 is the guarantee of protein efficient co-translation transport into the ER.As expected,knocking down SRP14 resulted in parts of PrPsp-NHK-GFP fusion protein rerouted to the cytosol.To clarify the effects of the decrease of SRP14 under ER stress,we overexpressed SRP14 in stressed cells and found overexpressed SRP14 could aggravate the burden and induce more cells death.We were wondering whether the degradation of SRP14 upon ER stress was under the control of UPR and found PERK knockdown could abolish the decrease of SRP14.Collectively,the multi-omics provide the clue between the ER stress and protein transport.Translocational attenuation regulated by PERK-SRP14 axis is a protective measure to cope with protein burden in stressed cells.The mechanism also provides a bridge between the classical signal peptide theory and UPR.