Final answer:
A virus has likely infected the cell and altered its gene expression, causing a cell surface signaling protein to be present in the nucleus.
Step-by-step explanation:
The most likely cause for a cell surface signaling protein being present in the nucleus instead of on the cell surface is that a virus has infected the cell and replaced some of its DNA. Viruses can interfere with the normal processes of a cell, including the synthesis, trafficking, and placement of proteins. In this case, the virus has likely altered the gene expression in the cell, leading to the relocation of the signaling protein to the nucleus. This alteration could be a result of the virus producing proteins that interfere with the cell's normal functioning.