The role of transmembrane domain III in the lactose permease of escherichia coli

• Published in: Protein science 1994-12, Vol.3 (12), p.2302-2310
• Main Authors: Sahin‐Tóth, Miklós, Frillingos, Stathis, Bibi, Eitan, Gonzalez, Alberto, Ronald Kaback, H.
• Format: Article
• Language: English
• Subjects: active transport; Amino Acid Sequence; Bacterial Proteins - antagonists & inhibitors; Bacterial Proteins - chemistry; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Base Sequence; bioenergetics; Biological Transport, Active - drug effects; Cys replacements; Cysteine - chemistry; C‐less permease; Escherichia coli - enzymology; Escherichia coli - genetics; Escherichia coli Proteins; Ethylmaleimide - pharmacology; Lactose - metabolism; lactose permease; Membrane Transport Modulators; Membrane Transport Proteins - antagonists & inhibitors; Membrane Transport Proteins - chemistry; Membrane Transport Proteins - genetics; Membrane Transport Proteins - metabolism; Models, Molecular; Molecular Sequence Data; Monosaccharide Transport Proteins; Mutagenesis, Site-Directed; Protein Structure, Tertiary; Recombinant Fusion Proteins - chemistry; Recombinant Fusion Proteins - metabolism; scanning mutagenesis; Structure-Activity Relationship; Symporters; Thiogalactosides - pharmacology
• ISSN: 0961-8368; 1469-896X
• DOI: 10.1002/pro.5560031215

Deletion of putative transmembrane helix III from the lactose permease of Escherichia coli results in complete loss of transport activity. Similarly, replacement of this region en bloc with 23 contiguous Ala, Leu, or Phe residues abolishes active lactose transport. The observations suggest that helix III may contain functionally important residues; therefore, this region was subjected to Cys‐scanning mutagenesis. Using a functional mutant devoid of Cys residues (C‐less permease) each residue from Tyr 75 to Leu 99 was individually replaced with Cys. Twenty‐one of the 25 mutants accumulate lactose to >70% of the steady‐state exhibited by C‐less permease, and an additional 3 mutants transport to lower, but significant levels (40–60% of C‐less). Cys replacement for Leu 76 results in low transport activity (18% of C‐less). However, when placed in the wild‐type background, mutant Leu 76 → Cys exhibits highly significant rates of transport (55% of wild type) and steady‐state levels of lactose accumulation (65% of wild type). Immunoblots reveal that the mutants are inserted into the membrane at concentrations comparable to wild type. Studies with N‐ethylmaleimide show that mutant Gly 96 → Cys is rapidly inactivated, whereas the other single‐Cys mutants are not altered significantly by the alkylating agent. Moreover, the rate of inactivation of Gly 96 → Cys permease is enhanced at least 2‐fold in the presence of β‐galactopyranosyl 1‐thio‐β,D‐galactopyranoside. The observations demonstrate that although no residue per se appears to be essential, structural properties of helix III are important for active lactose transport.