Expression cassette, recombinant host cell and process for producing a target protein

1. A process for producing a target protein, comprising:

providing a host cell having an outer membrane and harboring an expression cassette containing a recombinant polynucleotide therein, wherein the host cell is capable of expressing a fusion protein encoded by the recombinant polynucleotide and wherein the fusion protein comprises:

(i) an anchoring protein that comprises a N-terminal amino acid sequence of an ice nucleation protein, so that the fusion protein, once expressed in the host cell, is directed by the anchoring protein to be anchored and exposed on the outer membrane of the host cell;

(ii) the target protein; and

(iii) a self-splicing protein that comprises a first end fused with the anchoring protein and a second end fused with the target protein, wherein the self-splicing protein comprises a N-terminal or C-terminal amino acid sequence of an intein protein at the second end thereof, such that upon an environmental stimulus, the self-splicing protein exerts a self-cleavage at the second end thereof to release the target protein from the fusion protein;

obtaining a cell culture by culturing the host cell in a medium under a condition that enables the fusion protein to be expressed and anchored on the outer membrane of the host cell;

subjecting the cell culture thus obtained to an environmental stimulus that induces the self-splicing protein to exert a self-cleavage at the second end thereof; and

recovering the target protein by a separating treatment.

2. The process according to claim 1 , wherein in the fusion protein encoded by the recombinant polynucleotide, the anchoring protein is a full-length or truncated ice nucleation protein.

3. The process according to claim 1 , wherein in the fusion protein encoded by the recombinant polynucleotide, the self-splicing protein is an intein protein selected from the group consisting of Ssp DnaB intein, Ssp DnaE intein, Mxe GyrA intein, VMA intein, Mtu RecA intein, Psp Pol-I intein, PI-pful intein, PI-pfull intein, and Mth RIR1 intein.

4. The process according to claim 1 , wherein the environmental stimulus that induces the self-splicing protein to exert a self-cleavage is selected from the group consisting of a pH change, a temperature change, a salt concentration, or a combination thereof.

5. The process according to claim 4 , wherein the environmental stimulus that induces the self-splicing protein to exert a self-cleavage is a pH change, in which the cell culture is transferred from a first pH to a second pH, wherein the second pH is higher or lower than the first pH.

6. The process according to claim 5 , wherein the environmental stimulus that induces the self-splicing protein to exert a self-cleavage is a pH change, in which the cell culture is transferred from pH 8.5 to pH 7.5.

7. The process according to claim 5 , wherein the environmental stimulus that induces the self-splicing protein to exert a self-cleavage is a pH change, in which the cell culture is transferred from pH 7 to pH 10.

8. The process according to claim 4 , wherein the environmental stimulus that induces the self-splicing protein to exert a self-cleavage is a temperature change, in which the cell culture is transferred from a first temperature ranging from 15° C. to 37° C. to a second temperature ranging from 18° C. to 37° C., wherein the second temperature is higher than the first temperature.

9. The process according to claim 4 , wherein the environmental stimulus that induces the self-splicing protein to exert a self-cleavage is a temperature change, in which the cell culture is transferred from 18° C. to 37° C.

10. The process according to claim 1 , wherein the expression cassette further comprises a promoter sequence selected from the group consisting of T7 promoter, T5 promoter, lac promoter, tac promoter, T7 A1 promoter, trp promoter, trc promoter, araBAD promoter, and λP R P L promoter.

11. The process according to claim 1 , wherein the host cell is E. coli.

12. An expression cassette containing a recombinant polynucleotide encoding a fusion protein, wherein the fusion protein comprises:

(i) an anchoring protein that comprises a N-terminal amino acid sequence of an ice nucleation protein, so that the fusion protein, once expressed in a host cell transformed by the expression cassette, is directed by the anchoring protein to be anchored and exposed on the outer membrane of the host cell;

(ii) the target protein; and

(iii) a self-splicing protein that comprises a first end fused with the anchoring protein and a second end fused with the target protein, wherein the self-splicing protein comprises a N-terminal or C-terminal amino acid sequence of an intein protein at the second end thereof, such that upon an environmental stimulus, the self-splicing protein exerts a self-cleavage at the second end thereof to release the target protein from the fusion protein.

13. The expression cassette according to claim 12 , wherein in the fusion protein encoded by the recombinant polynucleotide, the anchoring protein is a full-length or truncated ice nucleation protein.

14. The expression cassette according to claim 12 , wherein in the fusion protein encoded by the recombinant polynucleotide, the self-splicing protein is an intein protein selected from the group consisting of Ssp DnaB intein, Ssp DnaE intein, Mxe GyrA intein, VMA intein, Mtu RecA intein, Psp Pol-I intein, PI-pful intein, PI-pfull intein, and Mth RIR1 intein.

15. The expression cassette according to claim 12 , further comprising a promoter sequence operatively connected to the recombinant polynucleotide.

16. The expression cassette according to claim 15 , wherein the promoter sequence is selected from the group consisting of T7 promoter, T5 promoter, lac promoter, tac promoter, T7 A1 promoter, tip promoter, trc promoter, araBAD promoter, and λP R P L promoter.

17. A recombinant host cell harboring an expression cassette containing a recombinant polynucleotide therein, wherein the host cell is capable of expressing a fusion protein encoded by the recombinant polynucleotide and wherein the fusion protein comprises:

(i) an anchoring protein that comprises a N-terminal amino acid sequence of an ice nucleation protein, so that the fusion protein, once expressed in the host cell, is directed by the anchoring protein to be anchored and exposed on the outer membrane of the host cell;

(ii) the target protein; and

(iii) a self-splicing protein that comprises a first end fused with the anchoring protein and a second end fused with the target protein, wherein the self-splicing protein comprises a N-terminal or C-terminal amino acid sequence of an intein protein at the second end thereof, such that upon an environmental stimulus, the self-splicing protein exerts a self-cleavage at the second end thereof to release the target protein from the fusion protein.

18. The recombinant host cell according to claim 17 , wherein in the fusion protein encoded by the recombinant polynucleotide, the anchoring protein is a full-length or truncated ice nucleation protein.

19. The recombinant host cell according to claim 17 , wherein in the fusion protein encoded by the recombinant polynucleotide, the self-splicing protein is an intein protein selected from the group consisting of Ssp DnaB intein, Ssp DnaE intein, Mxe GyrA intein, VMA intein, Mtu RecA intein, Psp Pol-I intein, PI-pful intein, PI-pfull intein, and Mth RIR1 intein.

20. The recombinant host cell according to claim 17 , further comprising a promoter sequence operatively connected to the recombinant polynucleotide.

21. The recombinant host cell according to claim 20 , wherein the promoter sequence is selected from the group consisting of T7 promoter, T5 promoter, lac promoter, tac promoter, T7 A1 promoter, tip promoter, trc promoter, araBAD promoter, and λP R P L promoter.

22. The recombinant host cell according to claim 17 , which is an E. coli cell.