The secondary structure of lactose permease (I) of Escherichia coli reconstituted in lipid membranes was detd. by Raman spectroscopy. The a-helix content was .apprx.70%, the b-strand content was <10%, and b-turns contributed 15%. About 1/3 of the residues in a-helixes and most other residues were exposed to water. Employing a method for structural prediction that accounts for amphipathic helixes, 10 membrane-spanning helixes were predicted that are either hydrophobic or amphipathic. They are expected to form an outer ring of helixes in the membrane. The interior of the ring would be made of residues that are predominantly hydrophilic and, evoking the analogy to sugar-binding proteins, suited to provide the sugar-binding site. [on SciFinder (R)]