Patent ID: 7666817

Claim:
A method for identifying a peptide substrate for an enzyme, comprising: contacting a cell with an enzyme, the cell comprising a cell outer membrane and expressing a peptide display scaffold, the peptide display scaffold comprising a fusion protein comprising the formula: wherein TM is a circularly permuted bacterial outer membrane protein; C 1 comprises a member of a library of candidate peptide substrates for the enzyme; and D 1 and D 2 are first and second detectable moieties, wherein D 1 and D 2 are not the same and wherein when D 1 provides a detectable D 1 signal D 2 does not provide a detectable D 2 signal above a background level of a detectable D 2 signal; and wherein D 1 -C 1 and D 2 are accessible at the extracellular surface of the cell outer membrane and the N- or C-terminus of the fusion protein is accessible at the extracellular surface of the cell outer membrane, wherein prior to contacting the cell with the enzyme, the cell exhibits a detectable D 1 signal; and detecting the presence or absence of a D 2 signal above a background level of a detectable D 2 signal, wherein the presence of a detectable D 2 signal above a background level of a detectable D 2 signal identifies the member of the library of candidate peptide substrates as a peptide substrate for the enzyme.