Lactose permease of Escherichia coli is a polytopic membrane transport protein containing 12 membrane-spanning segments. When the amino (N6)- and carboxy (C6)-terminal halves are expressed as separate gene fragments, association of the first half (N6) of the permease with the second half (C6) is necessary for stable insertion of C6 [Bibi, E., & Kaback, H. R. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 4325-4329]. In this report we demonstrate that N6-C6 interaction is specific, since N6 fragments derived from the structurally related tetracycline or sucrose transporters are unable to stabilize insertion of C6 from lactose permease. Furthermore, this association appears to be mediated by multiple transmembrane domains, since co-expression of progressively truncated N-terminal fragments (N5, N4, N3, N2, N1) with C6 leads to markedly decreased, but detectable amounts of C6 in the membrane. The results indicate that the N- and C-terminal six transmembrane domains of lactose permease are integrated into the membrane as separate units, and insertion of the C-terminal half is directed by specific interactions with the N-terminal half of the protein.