title: Dimerization of the transmembrane domain of Integrin alphaIIb subunit in cell membranes.
abstract: Homo- and hetero-oligomeric interactions between the transmembrane (TM) helices of integrin alpha and beta subunits may play an important role in integrin activation and clustering. As a first step to understanding these interactions, we used the TOXCAT assay to measure oligomerization of the wild-type alpha(IIb) TM helix and single-site TM domain mutants. TOXCAT measures the oligomerization of a chimeric protein containing a TM helix in the Escherichia coli inner membrane via the transcriptional activation of the gene for chloramphenicol acetyltransferase. We found the amount of chloramphenicol acetyltransferase induced by the wild-type alpha(IIb) TM helix was approximately half that induced by the strongly dimerizing TM helix of glycophorin A, confirming that the alpha(IIb) TM domain oligomerizes in biological membranes. Mutating each of the alpha(IIb) TM domain residues to either Ala, Leu, Ile, or Val revealed that a GXXXG motif mediates oligomerization. Further, we found that the residue preceding each glycine contributed to the oligomerization interface, as did the residue at position i + 4 after the second Gly of GXXXG. Thus, the sequence XXVGXXGGXXXLXX is critical for oligomerization of alpha(IIb) TM helix. These data were used to generate an atomic model of the alpha(IIb) homodimer, revealing a family of structures with right-handed crossing angles of 40 degrees to 60 degrees, consistent with a 4.0-residue periodicity, and with an interface rotated by 50 degrees relative to glycophorin A. Thus, although the alpha(IIb) TM helix makes use of the GXXXG framework, neighboring residues have evolved to engineer its dimerization interface, enabling it to subserve specific and specialized functions.
authors.affiliation: Department of Biochemistry and Biophysics and Hematology-Oncology Division, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, USA.
authors.forename: Renhao
authors.initials: R
authors.lastname: Li
authors.forename: Roman
authors.initials: R
authors.lastname: Gorelik
authors.forename: Vikas
authors.initials: V
authors.lastname: Nanda
authors.forename: Peter B
authors.initials: PB
authors.lastname: Law
authors.forename: James D
authors.initials: JD
authors.lastname: Lear
authors.forename: William F
authors.initials: WF
authors.lastname: DeGrado
authors.forename: Joel S
authors.initials: JS
authors.lastname: Bennett
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D002462:Cell Membrane / Q000378:metabolism*; D015500:Chloramphenicol O-Acetyltransferase / Q000235:genetics / Q000378:metabolism; D019281:Dimerization; D004591:Electrophoresis, Polyacrylamide Gel; D004797:Enzyme-Linked Immunosorbent Assay; D004926:Escherichia coli / Q000378:metabolism; D005816:Genetic Complementation Test; D005998:Glycine / Q000737:chemistry; D006021:Glycophorins / Q000737:chemistry; D008958:Models, Molecular; D015233:Models, Statistical; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D040201:Platelet Membrane Glycoprotein IIb / Q000737:chemistry*; D017434:Protein Structure, Tertiary; D015533:Transcriptional Activation; D014461:Ultracentrifugation
chemical_list: D006021:Glycophorins; D040201:Platelet Membrane Glycoprotein IIb; D015500:Chloramphenicol O-Acetyltransferase; D005998:Glycine