Abstract: The present invention concerns the use of a protein capable of spontaneously forming an isopeptide bond for the development of a peptide tag and binding partner pair wherein said peptide tag and binding partner are capable of covalently binding to each other via an isopeptide bond. The invention also provides a method for developing a peptide tag and binding partner pair which are capable of covalently binding to each other based on a protein which is capable of spontaneously forming an isopeptide bond. Additionally provided are peptide tag and binding partner pairs which are obtainable from isopeptide proteins. Further, specifically developed peptide tags and binding partners are encompassed, together with nucleic acid molecules and vectors which encode those peptides or proteins.

Abstract: The present invention provides a mutant streptavidin subunit which comprises one or more amino acid substitutions compared to a wildtype streptavidin subunit at any one of residue positions equivalent to positions (50, 51, 52, 53) and (54) of SEQ ID NO. 2 and wherein amino acid residues at positions equivalent to positions (23, 27, 43, 45, 49, 79, 88, 90, 92, 108, 110) and (128) of SEQ ID NO. 2, in said mutant streptavidin subunit, are wildtype, wherein (i) when said mutant streptavidin subunit unit is comprised in streptavidin, said streptavidin has a lower off rate for biotin or for a biotin conjugate than wildtype streptavidin or (ii) when said mutant streptavidin subunit is in monomelic form said mutant monomelic streptavidin has a lower off rate for biotin or a biotin conjugate than monomelic streptavidin. The invention also encompasses nucleic acid molecules comprising a nucleotide sequence encoding the mutant streptavidin subunit and vectors and cells comprising the nucleic acid.

Abstract: A controlled valency semiconductor nanocrystal can have a desired number of compounds associated with it. The semiconductor nanocrystal can have exactly one compound associated with it, and the compound can have exactly one binding site for an affinity target. The semiconductor nanocrystal can be used to image single copies of cell-surface proteins.

Abstract: The invention relates, in part, to monovalent avidin and streptavidin compositions. The invention also relates to methods of preparing and using monovalent avidin and streptavidin compositions. In some aspects of the invention, the compositions are monovalent avidin or monovalent streptavidin with a single femtomolar biotin-binding site.

Abstract: The invention relates, in part, to monovalent streptavidin compositions. The invention also relates to methods of preparing and using monovalent streptavidin compositions. In some aspects of the invention, the compositions are monovalent streptavidin with a single femtomolar biotin-binding site.