What do two-hybrid systems typically measure?

Two-hybrid systems are specifically designed to investigate protein-protein interactions within a cell. This technique relies on the use of fusion proteins, where two proteins of interest are fused to different halves of a reporter protein. When these two proteins interact, the reporter protein is reconstituted, leading to a detectable signal. This method allows researchers to identify and quantify interactions between proteins in a living cell, providing insights into various biological processes and pathways.

The other options do not align with the fundamental purpose of two-hybrid systems. For instance, measuring drug interactions with proteins or gene expression levels involves different experimental setups and methodologies. Similarly, assessing the concentration of metabolites pertains more to metabolic studies rather than directly evaluating protein-protein interactions. Therefore, the accurate representation of what two-hybrid systems measure is indeed the interactions of proteins using fusion proteins.