A protein gives a single band on SDS gel electrophoresis, as shown in lanes 1 and 2 below....

A protein gives a single band on SDS gel electrophoresis, as shown in lanes 1 and 2 below. There is little if any effect from adding β-mercaptoethanol (BME) in the gel; it anything, the protein runs a little bit slower. When treated with the proteolytic enzyme thrombin (see Chapter 5) and electrophoresis in the absence of BMF, the protein migrates a bit more rapidly (lane 3). But if BME is present, two much more rapidly migrating bands are found (lane 4). Explain these results in terms of a model for the protein.