Polypeptide signal sequences of modified fragments of human immunoglobulin heavy chain binding protein (Bip) are disclosed. Also disclosed are fusion proteins comprising a modified fragment of human immunoglobulin heavy chain binding protein (Bip) operably linked to a heterologous polypeptide. Also disclosed are protein expression vectors comprising a promoter operably linked to a first DNA sequence encoding a signal sequence comprising a modified fragment of human immunoglobulin heavy chain binding protein (Bip) and a second DNA sequence encoding a heterologous polypeptide fused in frame to the first DNA sequence. Further disclosed are methods of producing a polypeptide comprising expressing a fusion protein comprising a polypeptide signal sequences of modified fragments of human immunoglobulin heavy chain binding protein (Bip) operably linked to a heterologous polypeptide and recovering the heterologous polypeptide.