Final answer:
Upon the fusion of mouse and human cells, the marker proteins on their surfaces exhibit lateral mobility within the plasma membrane and intermingle, which can be visualized using fluorescently tagged antibodies, implying that protein markers fuse rather than denature, synthesize or separate.
Step-by-step explanation:
When a mouse cell and a human cell, each bearing distinct surface protein markers, are fused with polyethylene glycol and subsequently cooled to 0 degrees Celsius, the process of interest is fusion. At this temperature, the two types of protein markers remain intact but can blend through lateral diffusion within the hybrid cell's plasma membrane. This demonstrates the fluid nature of cell membranes and the mobility of membrane proteins, as evidenced by the experiment where the markers eventually intermix when observed under UV light with fluorescently labeled antibodies.