Final answer:
Proteins with a nuclear localization signal are recognized by the nuclear transport factor, which guides them to the nuclear pore. Following the interaction with cytoplasmic fibrils of the pore, ATP hydrolysis aids in active transport of the protein into the nucleus, against a concentration gradient.
Step-by-step explanation:
Protein Import into the Nucleus
In step 1 of protein import through a nuclear pore, when a protein complex forms, it involves the interaction of the protein with a nuclear localization signal. Proteins destined for the nucleus possess this signal sequence which is recognized by the nuclear transport factor. This factor aids in guiding the protein to the nuclear pore complex. Upon reaching the nuclear pore, the protein binds to the cytoplasmic fibrils of the pore, inducing a conformational change that allows the protein to enter the nucleus.
Once the protein is in proximity to the nuclear pore, the energy from ATP hydrolysis is used to actively transport the protein through the nuclear envelope against a concentration gradient. This is a crucial step for the protein to accumulate inside the nucleus where it can perform its function. The nuclear transport factor then recycles back to the cytoplasm to assist in the import of other proteins.