Leader peptidase is an integral, transmembrane protein of the plasma membrane of Escherichia coli. Its membrane assembly requires its internal, uncleaved signal sequence, its large periplasmic carboxyl-terminal region, and an apolar domain that is known as a "hydrophobic helper." We now show that the polar cytoplasmic domain of leader peptidase is a unique membrane assembly element, which we term a "translocation poison" sequence. This sequence is defined by its ability to block the action of a signal sequence that either precedes or follows it. To our knowledge, this is the first entirely polar topogenic element. Deletion analysis shows that the role of the leader peptidase hydrophobic helper sequence in its membrane assembly is to overcome the block to assembly caused by the poison sequence.
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