Process for producing recombinant protein using novel fusion partner

The present invention provides a method of producing polypeptide utilizing a fusion protein of A-B type in the following formula (I), by culturing transformed microorganism comprising DNA sequence encoding the desirable polypeptide; A-B (I). In the above formula (I), A is a fusion partner of 25 or more amino acid residues where aspartic and glutamic acid residues are incorporated to have a net negative charge of 30% or more, and B is the target protein to be produced. The target protein can be isolated from the fusion protein by employing enzymatic cleavage site etc. at the carboxyl-terminus of the fusion partner.