Protein scaffolds

1. A polypeptide display library comprising a recombinant polypeptide scaffold comprising:

(i) seven beta strand domains designated A, B, C, D, E, F, and G, each having at least 95% homology to the amino acids of:

a) SEQ ID NO:239 for the A beta strand, SEQ ID NO:229 for the B beta strand, SEQ ID NO:230 for the C beta strand, SEQ ID NO: 236 for the D beta strand, SEQ ID NO: 232 for the E beta strand, SEQ ID NO: 245 for the F beta strand and SEQ ID NO:246 for the G beta strand; or

b) SEQ ID NO:239 for the A beta strand, SEQ ID NO:229 for the B beta strand, SEQ ID NO:247 for the C beta strand, SEQ ID NO: 236 for the D beta strand, SEQ ID NO:232 for the E beta strand, SEQ ID NO:248 for the F beta strand, and SEQ ID NO:234 for the G beta strand;

(ii) linked to six loop regions such that a loop region connects each beta strand domain, is designated AB, BC, CD, DE, EF, and FG and corresponds to the cognate loop regions of SEQ ID NO:1, the cognate loop regions of SEQ ID NO: 1 consisting of residues:

a) from 11 to 17 inclusive in an AB loop;

b) from 23 to 31 inclusive in an BC loop;

c) from 39 to 45 inclusive in an CD loop;

d) from 51 to 56 inclusive in an DE loop;

e) from 23 to 31 inclusive in an EF loop;

f) from 23 to 31 inclusive in an FG loop,

at least one of said loop regions being a non-naturally occurring variant of the cognate loop region in SEQ ID NO: 1; and

(iii) a disulfide bond linking the F and G beta strand domains or linking the C and F beta strand domains.

2. The library of claim 1 , wherein said scaffold comprises at least two loop region sequences that vary by deletion, substitution or addition by at least one amino acid from the corresponding loop sequences in the naturally occurring protein sequence, and wherein said two loop region sequences are selected from the group consisting of BC/DE, BC/FG, DE/FG, AB/CD, AB/EF, and CD/ED loops.

3. The library of claim 1 , wherein said scaffold comprises at least three loop region sequences that vary by deletion, substitution or addition by at least one amino acid from the corresponding loop sequences in the naturally occurring protein sequence and wherein said loop region sequences comprise any three loops selected from the group consisting of the AB, BC, CD, DE, EF, and FG loops.

4. The polypeptide display library of claim 1 , wherein said scaffold is displayed on the surface of a ribosome, bacteriophage, virus, bacteria, or yeast.

5. The library of claim 1 , wherein said library has a sequence diversity of at least 10 6 .

6. A collection of isolated nucleic acid molecules encoding the library of claim 1 .

7. An expression vector operably linked to the nucleic acid molecules of claim 6 .

8. A method of obtaining a polypeptide scaffold that binds to a target, said method comprising (a) contacting a target ligand with the library of claim 1 under conditions that allow a scaffold:target ligand complex to form, and (b) obtaining from the complex, the scaffold that binds the target ligand.

9. The method of claim 8 , further comprising randomizing at least one loop of said scaffold of the protein obtained in step (b) to generate a further randomized scaffold and repeating steps (a) and (b) using said further randomized scaffold.

10. The method of claim 9 , wherein said method comprises at least two loops.

11. The method of claim 9 , wherein said method comprises at least three loops.

12. The method of claim 8 , further comprising randomizing at least one loop of said scaffold of the protein obtained in step (b) to generate a further randomized scaffold and repeating steps (a) and (b) using said further randomized scaffold, wherein said repetition of steps (a) and (b) further comprises contacting a target distinct from the target of said first operation of step (a) and (b).

13. The method of claim 8 , further comprising randomizing at least one loop of said scaffold of the protein obtained step (b), wherein said loop was not randomized in said library to generate a further randomized scaffold and repeating steps (a) and (b) using said further randomized scaffold.

14. The method of claim 8 , further comprising randomizing at least one loop of said scaffold of the protein obtained step (b), wherein said loop was not randomized in said library to generate a further randomized scaffold and repeating steps (a) and (b) using said further randomized scaffold, wherein said repetition of steps (a) and (b) further comprises contacting a target distinct from the target of said first operation of step (a) and (b).

15. The method of claim 8 , wherein said method further comprising randomizing at least one beta strand of said scaffold of the protein obtained in step (b) to generate a further randomized scaffold and repeating steps (a) and (b) using said further randomized scaffold.

16. The method of claim 15 , said method further comprising at least two, three, four, five, six or seven beta strands of said scaffold of the protein obtained in step (b) to generate a further randomized scaffold and repeating steps (a) and (b) using said further randomized scaffold.

17. A method of obtaining at least two scaffolds that bind to a target said method comprising (a) contacting a target ligand with the library of claim 1 under conditions that allow a scaffold:target ligand complex to form, (b) engaging said scaffolds with a crosslinking agent wherein the crosslinking of said scaffolds elicits a detectable response and (c) obtaining from the complex the scaffolds that bind the target.

18. The method of claim 17 wherein said scaffolds recognize the same epitope.

19. The method of claim 17 , wherein said scaffolds recognize distinct epitopes.

20. The method of claim 17 , wherein said crosslinking agent is selected from the group consisting of an antibody, an antibody fragment, a dimerization motif, a chemical crosslinker, a binding peptide, or an epitope tag.