The folding stability of insulin is positively correlated with the expression yield of the precursor expressed in yeast. Insulin assembles into dimers and hexamers in a concentration-dependent manner and amino acid substitutions that impair the ability of insulin to associate into dimers concomitantly decrease the expression yield (excluding substitutions that enhance folding stability). In contrast, introduction of an amino substitution that enhances the self-association of insulin improved the yeast expression yield. In the monomeric state the majority of the non-polar residues of insulin are exposed to the solvent and assembly into dimers and hexamers shields these from contact with the solvent. It is proposed that self-association enhances the flux of insulin through the secretory pathway by increasing the hydrophilicity, decreasing the surface area as well as decreasing the molar concentration in the secretory pathway. (C) 2002 Elsevier Science (USA). All rights reserved.