Polyvalent display libraries

DETAILED DESCRIPTION I.
General The present invention is directed to inter alia two related but self-sufficient improvements in conventional display methods.
The first improvement provides methods of enriching conventional display libraries for members displaying more than one copy of a polypeptide prior to affinity screening of such libraries with a target of interest.
Although practice of the claimed methods is not dependent on an understanding of mechanism, the rationale for these methods is believed to be that affinity selection of library members to immobilized binding partners occurs predominantly or exclusively through formation of multivalent bonds between multiple copies of displayed polypeptides on a library member and immobilized binding partners.
Accordingly, only members of library displaying multiple copies of a polypeptide are capable of surviving affinity selection to immobilized binding partners.
For example, conventional libraries of polypeptides fused to pVIII or a filamentous phage typically have Poisson distribution, in which most members display no copies of a polypeptide, a small proportion display one copy of a polypeptides, a still smaller proportion display two copies, and a still smaller proportion display three or more copies.
The methods of the present invention enrich for the small proportion of conventional display libraries displaying two or more copies of a polypeptide.
It is this rare fraction of conventional libraries that is capable of being affinity selected by immobilized binding partners.
Enrichment can be achieved by the inclusion of a tag as a component of the fusion protein from which polypeptides are displayed.
The tag can be any polypeptide with a known receptor showing high binding specificity for the tag.
The same tag is included in each member of the library.
Enrichment is effected by screening the library for affinity binding to an immobilized receptor for the tag.
Only library members having two or more copies of the tag are capable of binding to the immobilized receptor