Methods and compositions for protein and peptide sequencing

1. A method of sequencing a peptide, comprising:

(a) incubating the peptide with a library of DNA aptamers exhibiting binding specificity toward at least one N-terminal amino acid under conditions where one or more aptamers bind specifically to at least one N-terminal amino acid of the peptide, wherein each aptamer in the library comprises a peptide binding ssDNA region and a unique barcode sequence indicative of the first binding round and the associated peptide binding ssDNA region;

(b) ligating the DNA aptamer bound to the N-terminal of the peptide onto a proximal DNA barcode construct;

(c) removing the peptide binding sequence from the DNA aptamer, thereby leaving the barcode of the DNA aptamer covalently attached to the DNA barcode construct:

(d) removing the N-terminal amino acid from the peptide to produce an N-terminal amino acid-shortened peptide;

(e) incubating the N-terminal amino acid-shortened peptide with the library of aptamers exhibiting binding specificity toward at least one N-terminal amino acid under conditions where one or more aptamers bind specifically to at least one N-terminal amino acid of the N-terminal amino acid-shortened peptide, wherein each aptamer in the library comprises a peptide binding ssDNA region and a unique barcode sequence indicative of the second binding round and the associated peptide binding ssDNA region;

(f) ligating the DNA aptamer bound to the N-terminal of the peptide onto the proximal DNA barcode construct;

(g) removing the peptide binding sequence from the DNA aptamer, thereby leaving the barcode of the DNA aptamer covalently attached to the DNA barcode construct;

(h) removing the N-terminal amino acid from the N-terminal amino acid-shortened peptide;

(i) repeating steps (a)-(d) a plurality of times to construct a chain of positional barcodes that correspond to sequential N-terminal amino acids in the peptide; and

(j) sequencing the chain of positional barcodes to thereby obtain the sequence of the peptide.

2. The method of claim 1 , wherein the peptide is from a synthetic sample, biological sample, or combinations thereof.

3. The method of claim 2 , wherein the biological sample is selected from the group consisting of blood, urine, saliva, tissue biopsy, sputum, stool, single cell, environmental samples, and bacterial swabs.

4. The method of claim 1 , wherein the peptide is a full-length protein or a peptide comprised within a complex.

5. The method of claim 1 , further comprising, prior to step (a), fragmenting the peptide.

6. The method of claim 5 , wherein the fragmenting step comprises exposing the peptide with a fragmentation enzyme.

7. The method of claim 1 , wherein the C-terminal end of the peptide is attached to a solid support.

8. The method of claim 1 , wherein the C-terminal end of the peptide is attached to an oligonucleotide tail.

9. The method of claim 1 , wherein the removing the peptide binding sequence from the DNA aptamer step comprises cleaving the aptamer with a restriction enzyme.

10. The method of claim 1 , wherein the removing of the peptide binding sequence from the DNA aptamer is mediated by hydrogen bond disruption.

11. The method of claim 1 , wherein the removing the N-terminal amino acid step comprises Edman degradation of the peptide, cleaving the peptide with one or more aminopeptidases, heat, pH, or combinations thereof.

12. The method of claim 1 , wherein the sequencing step uses a next generation sequencing (NGS) platform.

13. A method of identifying novel biomarkers, comprising:

(a) providing protein samples from biological samples of interest and control or comparison biological samples;

(b) optionally, removing proteins present in high concentrations;

(c) performing the steps (a)-(j) of the method of claim 1 ;

(d) performing a sup-diff to remove high concentration DNA barcode construct sequences associated with highly expressed proteins or contaminants such that a ratio of DNA barcode constructs associated with lowly expressed proteins to highly expressed proteins increases, thereby producing ratio-adjusted DNA barcodes;

(e) amplifying the ratio-adjusted DNA barcodes; and

(f) comparing the number of sequencing reads associated with each lowly expressed proteins from control samples to samples of interest;

thereby identifying biomarkers that have different relative expression levels between the control samples and the biological samples of interest.

14. A method of evaluating a disease state, evaluating a response to treating a disease, predicting a response to treating a disease, or combinations thereof, wherein the disease comprises aberrant expression levels of at least one known protein biomarker, the method comprising:

(a) providing protein samples from patient samples;

(b) optionally depleting proteins present in high concentrations;

(c) performing the steps (a)-(j) of the method of claim 1 ;

(d) performing a sup-diff to remove high concentration DNA barcode construct sequences associated with highly expressed proteins or contaminants to increase the ratio of DNA barcode constructs associated with lowly expressed proteins to highly expressed proteins, thereby producing ratio-adjusted DNA barcodes;

(e) amplifying the ratio-adjusted DNA barcodes;

(f) analyzing the number of sequencing reads associated with known protein biomarkers to determine the relative quantity thereof;

(g) determining the presence or absence of expression level deviations of known protein biomarkers from one or more standard values,

thereby evaluating the disease state, evaluating the response to treating the disease, predicting the response to treating a disease, or combinations thereof.

15. The method of claim 1 , wherein the library of aptamers is produced using a RCHT-SELEX method.

16. The method of claim 1 , wherein each aptamer in the library of aptamers exhibits binding specificity toward one N-terminal amino acid.

17. The method of claim 1 , wherein each aptamer in the library of aptamers exhibits binding specificity toward two or more N-terminal amino acids.

18. The method of claim 1 , wherein the unique barcode sequence indicative of the aptamer's associated peptide binding ssDNA region and binding round comprises about 6 to about 20 nucleotides.

19. The method of claim 1 , wherein each aptamer in the library of aptamers comprises a BCS compatible portion.

20. The method of claim 19 , wherein the BCS compatible portion of each aptamer comprises one or more DNA sequences complementary to the aptamer.

21. The method of claim 1 , wherein the barcode sequence comprises a unique barcode that indicates the peptide or the sample from which the peptide was derived.