title: Alternating-access mechanism in conformationally asymmetric trimers of the betaine transporter BetP.
abstract: Betaine and Na(+) symport has been extensively studied in the osmotically regulated transporter BetP from Corynebacterium glutamicum, a member of the betaine/choline/carnitine transporter family, which shares the conserved LeuT-like fold of two inverted structural repeats. BetP adjusts its transport activity by sensing the cytoplasmic K(+) concentration as a measure for hyperosmotic stress via the osmosensing carboxy-terminal domain. BetP needs to be in a trimeric state for communication between individual protomers through several intratrimeric interaction sites. Recently, crystal structures of inward-facing BetP trimers have contributed to our understanding of activity regulation on a molecular level. Here we report new crystal structures, which reveal two conformationally asymmetric BetP trimers, capturing among them three distinct transport states. We observe a total of four new conformations at once: an outward-open apo and an outward-occluded apo state, and two closed transition states--one in complex with betaine and one substrate-free. On the basis of these new structures, we identified local and global conformational changes in BetP that underlie the molecular transport mechanism, which partially resemble structural changes observed in other sodium-coupled LeuT-like fold transporters, but show differences we attribute to the osmolytic nature of betaine, the exclusive substrate specificity and the regulatory properties of BetP.
authors.affiliation: Max-Planck Institute of Biophysics, Department of Structural Biology, D-60438 Frankfurt am Main, Germany.
authors.forename: Camilo
authors.initials: C
authors.lastname: Perez
authors.forename: Caroline
authors.initials: C
authors.lastname: Koshy
authors.forename: Ozkan
authors.initials: O
authors.lastname: Yildiz
authors.forename: Christine
authors.initials: C
authors.lastname: Ziegler
mesh_terms: D001059:Apoproteins / Q000737:chemistry / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D001622:Betaine / Q000737:chemistry / Q000378:metabolism*; D001665:Binding Sites; D001692:Biological Transport; D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D048230:Corynebacterium glutamicum / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003593:Cytoplasm / Q000378:metabolism; D050485:GABA Plasma Membrane Transport Proteins; D008958:Models, Molecular; D019897:Periplasm / Q000378:metabolism; D050480:Plasma Membrane Neurotransmitter Transport Proteins / Q000737:chemistry; D011487:Protein Conformation; D017510:Protein Folding; D055503:Protein Multimerization; D012964:Sodium / Q000378:metabolism; D013329:Structure-Activity Relationship; D027981:Symporters
chemical_list: D001059:Apoproteins; D001426:Bacterial Proteins; C101652:BetP protein, Corynebacterium glutamicum; D002352:Carrier Proteins; D050485:GABA Plasma Membrane Transport Proteins; D050480:Plasma Membrane Neurotransmitter Transport Proteins; D027981:Symporters; C072207:betaine plasma membrane transport proteins; D001622:Betaine; D012964:Sodium