What happens to eIF2B when there is more eIF2 than eIF2B present in the cell?

eIF2B is a guanine nucleotide exchange factor (GEF) that plays a crucial role in the initiation of translation by facilitating the exchange of GDP for GTP on eIF2, which is necessary for the assembly of the translation initiation complex. When there is an abundance of eIF2 relative to eIF2B in the cell, eIF2 can bind more preferentially to and sequester eIF2B. This sequestering reduces the availability of eIF2B to perform its role in facilitating the exchange process.

When eIF2 occupies eIF2B's binding sites, the eIF2B cannot engage effectively in promoting translation, leading to a potential decrease in translation initiation. The excess of eIF2 effectively outcompetes eIF2B for binding, thereby preventing the exchange of nucleotides that would normally regenerate active eIF2 for translation initiation.

This is an important regulatory mechanism in the cell, as the balance between these factors is critical for managing protein synthesis in accordance with cellular needs.