Interacts with target proteins during their translocation into the lumen of the endoplasmic reticulum. Protects unfolded target proteins against degradation during ER stress. May facilitate glycosylation of target proteins after termination of ER stress. May modulate the use of N-glycosylation sites on target proteins. . This is the function of SERP1 (stress associated endoplasmic reticulum protein 1, ENSG00000120742).