As of2011, approximately 336 million people worldwide have diabetes-most needing to produce more insulin because of insulin resistance, yet nearly all with pancreatic β-cell dysfunction.In β-cells, insulin biosynthesis begins with the precursor, preproinsulin, which must undergo co-translational translocation into endoplasmic reticulum (ER), signal peptide cleavage, and downstream proinsulin folding.To homeostatically maintain the insulin storage pool, β-cells must synthesize large amount of proinsulin, whose biosynthesis accounts for 6-10% (under basal) and up to 30-50% (under stimulated) of the total protein synthesis in the β-cells.This puts enormous pressure on the proinsulin folding machinery-ER.Accumulating evidence suggests that, under physiological conditions, up to 20% of newly synthesized proinsulin fails to achieve its native structure in the ER.Therefore, β-cells must continuously refold and/or dispose misfolded proinsulin.Under certain pathological conditions, the misfolded proinsulin increases exceeding the folding capacity that β-cells are genetically determined, resulting in accumulation of misfolded proinsulin in the ER, leading to β-cell failure.However, the molecular mechanisms of β-cell failure caused by misfolded proinsulin remain unknown.Over past four years, more than twenty new insulin gene mutations have been found to underlie a syndrome we call Mutant INS-gene-induced Diabetes of Youth (MIDY), in which heterozygotes develop autosomal dominant diabetes.These mutations cause proinsulin misfolding and therefore provide useful tools to study β-cell failure caused by misfolded proinsulin.It has been shown that mutants induce ER stress and β-cell death.However, recent studies find that the insulin deficiency in mice carrying a MIDY mutant precedes decreased β-cell mass, suggesting that other mechanism(s) may account for initiation of diabetes caused by misfolded proinsulin.We now show that misfolded mutant proinsulin not only fails to exit ER, but also impairs normal folding and trafficking of co-expressed wild-type proinsulin, decreasing insulin production, and initiating insulin deficient diabetes.Upon increased expression of ER-oxidoreductin-1 (ERO1), the oxidative folding of wild-type proinsulin is improved, leading to increased ER export and insulin production.Our studies shed new light on the prevention of β-cell failure caused by misfolded proinsulin.