Proper insertion of a complex membrane protein in the absence of its amino-terminal export signal.

The MalF protein spans the Escherichia coli cytoplasmic membrane eight times. Deletion of the first transmembrane stretch of MalF, which acts as an export signal, results in a truncated protein that still exhibits high levels of maltose transport activity. These and additional results indicate that the orientation of a membrane protein is not determined by the amino-terminal export signal, topological information is distributed throughout the MalF protein, and insertion of a protein into the cytoplasmic membrane can occur nonsequentially.