Fragment pairs of a Class A beta-lactamase (TEM-1 of E. coli) are disclosed that depend for their functional reassembly into the parent 
protein on the interaction of 
heterologous polypeptides or other molecules which have been genetically or chemically conjugated to the break-point termini of the fragment pairs. In addition, methods are provided for identifying fragment pairs that will optimally reassemble into a functional parent 
protein. Fragment pairs that comprise molecular interaction-dependent enzymes find use in (1) homogeneous assays and biosensors for any 
analyte having two or more independent binding sites, (2) tissue-localized activation of therapeutic and imaging reagents 
in vivo for 
early detection and treatment of 
cancer, chronic 
inflammation, atherosclerosis, 
amyloidosis, infection, 
transplant rejection, and other pathologies, (3 
cell-based sensors for activation or inhibition of metabolic or 
signal transduction pathways for high-efficiency, high-
throughput screening for agonists / antagonists of the target pathway, (4) high-
throughput mapping of pair-wise 
protein-protein interactions within and between the proteomes of cells, tissues, and pathogenic organisms, (5) rapid selection of 
antibody fragments or other binding proteins which bind specifically to polypeptides of interest, (6) rapid 
antigen identification for anti-
cell and anti-tissue antibodies, (7) rapid 
epitope identification for antibodies, (10) 
cell-based screens for high-
throughput selection of inhibitors of any protein-protein interaction.