A protein that is normally a single-pass transmembrane protein is absent from the cell surface of the mutant cell line YTM-15. When labeled so that the protein can be localized, you find that this protein is now secreted by the cell. Analysis of the amino acid sequence of the transmembrane protein in question reveals sequence changes that could lead to the observed phenotype. The amino acid substitutions lead to alterations in the ER signal sequence and the stop-transfer sequence. In addition, YTM-15 cells exhibit a point mutation in the gene encoding signal peptidase. Only one mutation is consistent with the observed phenotype. What is the underlying cause of the mutant trait? Explain your reasoning.