Polypeptide linker and method of analyzing target material using the same

1. A method of detecting a target material in a sample, the method comprising:

(a) incubating a sample with

(i) an antibody that binds to a target material in the sample, wherein the antibody comprises an Fc region, and

(ii) a polypeptide linker comprising an Fc-binding region and an enzyme cleavage region, wherein a microparticle is coupled to the polypeptide linker, to form a complex;

(b) isolating the complex based on the physical or chemical properties of the microparticle;

(c) incubating the complex with an enzyme that recognizes the enzyme cleavage region to cut the polypeptide linker at the enzyme cleavage region, which results in separation of the microparticle from the complex; and

(d) detecting the complex without the microparticle, thereby detecting the target material in the sample.

2. The method of claim 1 , wherein the sample is a biological sample.

3. The method of claim 1 , wherein the sample is blood, bone marrow, lymph, saliva, lachrymal fluid, urine, mucous, amniotic fluid, or a combination thereof.

4. The method of claim 1 , wherein the antibody is an antibody fragment.

5. The method of claim 1 , wherein the target material is a cell.

6. The method of claim 5 , wherein the target material is a tumor cell.

7. The method of claim 5 , wherein the target material is a circulating tumor cell.

8. The method of claim 1 , wherein the microparticle is a polymer bead or a magnetic bead.

9. The method of claim 1 , further comprising

(a1) washing after the formation of the complex,

(a2) washing after incubating the complex and the enzyme, or

(a3) a combination of (a1) and (a2).

10. The method of claim 1 , wherein the enzyme cleavage region comprises a protease recognition sequence.

11. The method of claim 10 , wherein the protease recognition sequence is a sequence recognized by tobacco etch virus (TEV) protease, trypsin, chymotrypsin, elastase, pepsin, enteropeptidase, or a combination thereof.

12. The method of claim 1 , wherein the polypeptide linker further comprises at least one spacer polypeptide positioned between the Fc-binding region and the enzyme cleavage region, at the end of the enzyme cleavage region opposite the Fc-binding region, or both.

13. The method of claim 1 , wherein the polypeptide linker further comprises an ubiquitin.

14. The method of claim 1 , wherein the polypeptide linker further comprises a fluorescent label.

15. The method of claim 14 , wherein the fluorescent label is fluorescein iso-thio-cyanate (FITC).