Improved methods of determining the intracellular state of a protein as well as modifications of the protein are provided by introducing a surrogate fusion protein comprising a member of an enzyme fragment complementation complex and a target protein. After exposing cells transformed with the surrogate fusion protein to a change in environment, e.g. a candidate drug, the cells are lysed, the lysate separated into fractions or bands, conveniently by gel electrophoresis and transferring the proteins by Western blot to a membrane. The bands on the membrane are developed using the other member of the enzyme fragment complementation complex and a substrate providing a detectable signal. The method is found to provide high sensitivity and the ability to observe modifications of the target protein.