The present invention provides functional annotation of novel genes by detection of interactions of their encoded proteins with known proteins followed by assays to validate that the gene participates in a specific cellular function. The instant invention also provides an experimental strategy that allows for detection of protein interactions and functional assays with a single reporter system. Interactions among network component proteins are detected and probed with stimulators and inhibitors of the network and subcellular location of the interacting proteins is determined. Additionally, applicants' use this strategy to map a signal transduction network that controls the G0 to G1 transition in eukaryotes. Analysis of 148 combinations of 65 protein pairs in mammalian cells allows applicants' to propose a model of network architecture. The results demonstrate the feasibility of employing this strategy in genome-wide functional annotation efforts.