The contribution of surface residues to membrane binding and ligand transfer by the α-tocopherol transfer protein (α-TTP).
Previous work has shown that the α-tocopherol transfer protein (α-TTP) can bind to vesicular or immobilized phospholipid membranes. Revealing the molecular mechanisms by which α-TTP associates with membranes is thought to be critical to understanding its function and role in the secretion of tocopherol from hepatocytes into the circulation. Calculations presented in the Orientations of Proteins in Membranes database have provided a testable model for the spatial arrangement of α-TTP and other CRAL-TRIO family proteins with respect to the lipid bilayer. These calculations predicted that a hydrophobic surface mediates the interaction of α-TTP with lipid membranes. To test the validity of these predictions, we used site-directed mutagenesis and examined the substituted mutants with regard to intermembrane ligand transfer, association with lipid layers and biological activity in cultured hepatocytes. Substitution of residues in helices A8 (F165A and F169A) and A10 (I202A, V206A and M209A) decreased the rate of intermembrane ligand transfer as well as protein adsorption to phospholipid bilayers. The largest impairment was observed upon mutation of residues that are predicted to be fully immersed in the lipid bilayer in both apo (open) and holo (closed) conformations such as Phe165 and Phe169. Mutation F169A, and especially F169D, significantly impaired α-TTP-assisted secretion of α-tocopherol outside cultured hepatocytes. Mutation of selected basic residues (R192H, K211A, and K217A) had little effect on transfer rates, indicating no significant involvement of nonspecific electrostatic interactions with membranes.
Department of Chemistry, Brock University, St. Catharines, Ontario, Canada L2S3A1.
Wen Xiao
WX
Zhang
Varsha
V
Thakur
Andrei
A
Lomize
Irina
I
Pogozheva
Candace
C
Panagabko
Matt
M
Cecchini
Matilda
M
Baptist
Samantha
S
Morley
Danny
D
Manor
Jeffrey
J
Atkinson
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D002352:Carrier Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D017931:DNA Primers / Q000235:genetics; D056945:Hep G2 Cells; D022781:Hepatocytes / Q000378:metabolism; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D066298:In Vitro Techniques; D008024:Ligands; D008051:Lipid Bilayers / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D050505:Mutant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011487:Protein Conformation; D054730:Protein Interaction Domains and Motifs; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D055672:Static Electricity; D013816:Thermodynamics; D024502:alpha-Tocopherol / Q000378:metabolism
D002352:Carrier Proteins; D017931:DNA Primers; D008024:Ligands; D008051:Lipid Bilayers; D050505:Mutant Proteins; D011994:Recombinant Proteins; C017763:alpha-tocopherol transfer protein; D024502:alpha-Tocopherol