Determining the topology of an integral membrane protein.

A variety of methods have been developed for assigning the aqueous domains of integral membrane proteins to either side of a biological membrane. Once the sequence of a protein is known from its DNA sequence it is possible to study the topology of the protein. This unit provides protocols in which the water-soluble domains can be tested for their accessibility to reagents added to membranes with a defined orientation. Tagging of hydrophilic regions of the protein with different epitopes and probing of their orientation with respect to the membrane is also described. Finally, a procedure for fusion of a reporter enzyme to truncated fragments of the protein is provided. The fusion protein is used as a sensor of sequence disposition relative to the membrane.