Chapter 6, Problem 14P

A protein gives a single band on SDS get electrophoresis, as shown in lanes 1 and 2 below. There is little, if any, effect from adding $\text{β}$ -mercaptoethanol (BME) to the sample; if anything, the protein runs a little bit slower. When treated with the proteotylic enzyme thrombin (see Chapter 5) and electrophoresis in the absence of BME, 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.