<s>[INST] A cell biologist is studying a protein that is involved in the movement of vesicles within the cell. The movement is mediated by a motor protein that binds to both the vesicle and a microtubule. In a healthy cell, the protein facilitates the movement of the vesicle along the microtubule. In an unhealthy cell, the protein is unable to bind to the microtubule, and the vesicle is unable to move.

The cell biologist uses a fluorescently tagged version of the protein to study its movement within the cell. However, the tagged protein is unable to bind to microtubules, and the vesicles are unable to move. The cell biologist decides to make a point mutation in the protein to try to restore its ability to bind to microtubules.

The cell biologist makes a point mutation in the protein that changes a serine residue to a glutamic acid residue. The protein with the mutation is able to bind to microtubules, but the movement of the vesicle is still impaired.

Which of the following best explains why the movement of the vesicles is still impaired despite the protein's ability to bind to microtubules?[/INST]