Process for the production of naturally folded and secreted proteins

A process for producing a water-soluble, naturally-folded eukaryotic polypeptide containing at least two cysteines linked by disulfide bridges, which comprises

• (a) culturing in a nutrient medium prokaryotic cells which contain an expression vector that (i) encodes the polypeptide and (ii) contains a prokaryotic signal sequence at its N-terminus, the culturing being under conditions such that the polypeptide is produced in soluble form and secreted into the periplasm of the prokaryotic cells or into the medium, the culturing being in the presence of an amount of arginine or a compound of the formula:
• R2—CO—NRR1 (I)
• wherein
• R and R1 are each independently hydrogen or a saturated or unsaturated, branched or unbranched C1-C4 alkyl chain, and
• R2 is hydrogen, NHR1, or a saturated or unsaturated, branched or unbranched C1-C3 alkyl chain,
• the amount of arginine or the compound of formula I being sufficient to minimize the formation of inclusion bodies;
• (b) cleaving the signal sequence from the soluble polypeptide;
• and
• (c) isolating the soluble polypeptide.