What does the binding of GAL3 to GAL80 prevent?

The binding of GAL3 to GAL80 prevents the activation of GAL4, which is key for the regulation of gene expression in the GAL (galactose) metabolic pathway in yeast. In this context, GAL3 acts as a sensor for galactose and binds to GAL80 when galactose is present. GAL80 typically functions as a repressor of GAL4, inhibiting its ability to activate transcription of genes necessary for galactose metabolism.

When GAL3 binds to GAL80, it leads to a conformational change in GAL80 that releases its inhibition on GAL4. As a result, GAL4 can then interact with the appropriate enhancers to drive the transcription of downstream genes that facilitate the utilization of galactose. This mechanism highlights the intricate regulation of gene expression through protein-protein interactions, where the presence of a molecule (in this case, galactose) can alter the functional dynamics among regulatory proteins, ultimately influencing gene activation in response to environmental changes.