What does the nuclear import receptor bind to in the cytosol to facilitate transport?

The nuclear import receptor binds to the nuclear localization signal (NLS) in the cytosol to facilitate transport into the nucleus. The nuclear localization signal is a specific amino acid sequence that marks proteins for entry into the nucleus. When a protein straddles the cytosol and nucleus, the nuclear import receptor recognizes and binds to this signal, allowing the complex to interact with the nuclear pore complex and facilitate import into the nucleus.

The process of nuclear import is dependent on the interaction between the import receptor and the NLS, which enables the receptor to escort the protein through the pore. Once inside the nucleus, the import receptor undergoes a conformational change and releases the protein, often facilitated by the presence of Ran-GTP, a small GTPase that helps to drive the release of the cargo protein. This sequence of actions ensures that proteins that require nuclear localization are successfully transported into the nucleus, where they can perform their functions.

While nucleoporins form part of the nuclear pore complex and are involved in the transport process, they do not act as the direct binding partner for the import receptor. Rather, they serve as structural components that form the gateways through which the transport occurs.