Which process do importins and exportins primarily facilitate?

Importins and exportins are types of transport proteins that play a crucial role in the nuclear transport system of cells. Specifically, they facilitate the transport of proteins and other macromolecules into and out of the nucleus. Importins bind to proteins that contain a nuclear localization signal, allowing these proteins to be translocated into the nucleus, where they can perform various functions related to gene expression and regulation. Conversely, exportins facilitate the export of proteins that contain a nuclear export signal from the nucleus back to the cytoplasm.

This process is critical for maintaining the proper cellular function and ensuring that proteins are appropriately located where they are needed, such as enzymes, transcription factors, and other regulatory molecules. Therefore, the primary function of importins and exportins is clearly aligned with protein trafficking, distinguishing it from other processes such as cell division, cell signaling, or metabolism.