The most likely defect associated with the ser63 deletion

Q. Unlike the wild-type P0 protein, which is localized to the cell surface, the Ser63 deletion P0 mutant is exclusively located at the ER. Based on these observations, what is the most likely defect associated with the Ser63 deletion mutant?
How would you design an experiment to test your hypothesis using culture cells and the expression construct of wild-type and Ser63 deletion mutant of P0?