Topological determinants of internal transmembrane segments in P-glycoprotein sequences.

P-glycoprotein (Pgp) is a polytopic membrane protein responsible for multidrug resistance in cancer cells. Previously, we have used a coupled cell-free translation/translocation system to investigate the membrane orientation of Pgp sequences and have made the unexpected observation that predicted transmembrane (TM) segments from both the NH2-terminal and COOH-terminal halves inserted in microsomal membranes in two different orientations (Zhang, J.-T., Duthie, M., and Ling, V. (1993) J. Biol. Chem. 268, 15101-15110). How these topological forms of Pgp are regulated is not known. In the present study, we have used site-directed mutagenesis to investigate if the amino acids surrounding the internal TM segments of Pgp may affect their orientation. We discovered that the charged amino acids flanking TM4 are important in determining the membrane orientation of the NH2-terminal half molecule of Pgp. This is a novel observation demonstrating the existence of internal topogenic sequences in a mammalian polytopic membrane protein. These findings thus suggest A) that the topological structure of a mammalian polytopic membrane protein does not integrate into the membrane simply by following the lead of the first inserted TM segment but that internal TMs may have independent topogenic information and B) that the TM segments in a multi-spanning membrane protein may be more dynamic than have been previously anticipated, i.e. mutations in the amino acids surrounding internal TMs could drastically change the overall topology of the molecule.