Which technique is NOT typically used to study protein interactions?

Substrate competition is not typically used to study protein interactions because it focuses on how substrates compete for binding to enzymes rather than directly analyzing the interactions between proteins themselves. This technique is often utilized to understand enzyme kinetics and substrate inhibition rather than to observe or quantify the physical interactions between proteins.

In contrast, fluorescent resonance energy transfer (FRET) is a powerful method used to study protein interactions at close range, as it relies on energy transfer between fluorescently tagged molecules. Cross-linking assays chemically link proteins that are in close proximity to study their interactions, and co-immunoprecipitation allows for the isolation of a particular protein and its binding partners from a complex mixture, revealing direct interactions. These methods are specifically designed to identify and characterize protein interactions, making substrate competition distinct in its application.