K+-transport protein TrkA of Escherichia coli is a peripheral membrane protein that requires other trk gene products for attachment to the cytoplasmic membrane

The TrkA protein, which is essential for the activity of the constitutive Trk K+-uptake system of Escherichia coli, is a peripheral membrane protein. The protein was detected in immunoblots by polyclonal antibodies to sodium dodecyl sulfate-denatured TrkA protein. In extracts from wild-type cells equal amounts of TrkA were found in the membrane and soluble fractions, suggesting that membrane binding is relatively weak. When the protein was moderately overproduced it appeared mainly in the soluble fraction; stronger overproduction led to the formation of aggregates that could not be solubilized by nonionic detergents. Mutations in the three other genes implicated in Trk activity, trkE, trkG, and trkH, reduced or abolished the binding of TrkA to the membrane. These results support the model, previously based solely on genetic data, that Trk is a multisubunit complex and implicates the products of the other trk genes in the normal binding of TrkA to the complex in the cytoplasmic membrane.