Production of proteins and polypeptides

A method of producing a desired protein or a desired polypeptide, comprising the following steps:

• a) expressing in a suitable host a fusion protein comprising
• (i) at least one solubility-enhancing moiety which is derived from the N-terminal (NT) fragment of a spider silk protein, wherein each solubility-enhancing moiety has at least 80% identity to the amino acid sequence of SEQ ID NO:
• 6 or at least 50% identity to the amino acid sequence of SEQ ID NO:
• 8;
• (ii) at least one moiety which is the desired protein or the desired polypeptide, wherein the desired protein or the desired polypeptide has less than 30% identity to any of the amino acid sequences of SEQ ID NO:
• 6-10;
• and
• (iii) at least one cleavage site arranged between the at least one desired protein moiety or desired polypeptide moiety and the at least one solubility-enhancing moiety;
• and
• wherein each solubility-enhancing moiety is linked directly or indirectly to the desired protein or desired polypeptide moiety;
• and
• b) obtaining a mixture containing the fusion protein, and optionally isolating the fusion protein;
• c) cleaving the fusion protein to provide the desired protein or the desired polypeptide;
• and
• d) isolating the desired protein or the desired polypeptide.