Sulfatide promotes the folding of proinsulin, preserves insulin crystals, and mediates its monomerization

Sulfatide is a glycolipid that has been associated with insulin-dependent d iabetes mellitus, It is present in the islets of Langerhans and follows the same intracellular route as insulin. However, the role of sulfatide in the beta cell has been unclear. Here we present evidence suggesting that sulfa tide promotes the folding of reduced proinsulin, indicating that sulfatide possesses molecular chaperone activity. Sulfatide associates with insulin b y binding to the insulin domain AS-BIG and most likely by interacting with the hydrophobic side chains of the dimer-forming part of the insulin B-chai n, Sulfatide has a dual effect on insulin. It substantially reduces deterio ration of insulin hexamer crystals at pH 5.5, conferring stability comparab le to those in beta cell granules. Sulfatide also mediates the conversion o f insulin hexamers to the biological active monomers at neutral pH, the pH at the beta-cell surface. Finally, we report that inhibition of sulfatide s ynthesis with chloroquine and fumonisine B1 leads to inhibition of insulin granule formation in vivo. Our observations suggest that sulfatide plays a key role in the folding of proinsulin, in the maintenance of insulin struct ure, and in the monomerization process.