Projection structure at 8 A resolution of the melibiose permease, an Na-sugar co-transporter from Escherichia coli.
The ion-coupled sugar membrane symporter or co-transporter melibiose permease (MelB), responsible for alpha-galactoside accumulation in Escherichia coli, is a representative member of the glycoside-pentoside- hexuronide family of the vast class of electrochemical potential-driven porters. Pure solubilized preparations of a MelB recombinant protein were subjected to two-dimensional crystallization trials and several crystal forms were observed. Two of these appeared as large wide tubes suitable for analysis by electron crystallography. Flattened tubes on carbon support film, embedded in amorphous ice prior to electron cryomicroscopy, showed two-sided plane group symmetries P12(1) or P222(1), with unit cell dimensions a = 89.9 A, b = 51.6 A, gamma = 91.9 degrees and a = 188.9 A, b = 48.8 A, gamma = 90 degrees, respectively. The projection map from the P222(1 )crystals at 8 A resolution displayed an asymmetric protein unit consisting of two domains lining a central and curve-shaped cleft. Together, the MelB monomer could host the 12 predicted transmembrane alpha-helices. Overall, the MelB helix packing arrangement compared more favorably with that of the Na(+)/H(+) antiporter NhaA than that of the oxalate antiporter.
Karolinska Institutet, Department of Biosciences, S-141 57 Huddinge, Sweden.
Ingrid
I
Hacksell
Jean-Louis
JL
Rigaud
Pasi
P
Purhonen
Thierry
T
Pourcher
Hans
H
Hebert
Gérard
G
Leblanc
D003461:Crystallography; D004926:Escherichia coli / Q000201:enzymology*; D005583:Fourier Analysis; D011487:Protein Conformation; D027981:Symporters / Q000737:chemistry*
D027981:Symporters; C021548:melibiose permease