A Structural Model for the Osmosensor, Transporter, and Osmoregulator ProP of Escherichia coli
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- 作者：Janet M. Wood ; Doreen E. Culham ; Alexander Hillar ; Yaroslava I. Vernikovska ; Feng Liu ; Joan M. Boggs ; and Robert A. B. Keates
- 刊名：Biochemistry
- 出版年：2005
- 出版时间：April 19, 2005
- 年：2005
- 卷：44
- 期：15
- 页码：5634 - 5646
- 全文大小：716K
- 年卷期：v.44,no.15(April 19, 2005)
- ISSN：1520-4995
文摘
Transporter ProP of Escherichia coli, a member of the major facilitator superfamily (MFS),acts as an osmosensor and an osmoregulator in cells and after purification and reconstitution inproteoliposomes. H+-osmoprotectant symport via ProP is activated when medium osmolality is elevatedwith membrane impermeant osmolytes. The three-dimensional structure of ProP was modeled with thecrystal structure of MFS member GlpT as a template. This GlpT structure represents the inward (orcytoplasm)-facing conformation predicted by the alternating access model for transport. LacZ-PhoA fusionanalysis and site-directed fluorescence labeling substantiated the membrane topology and orientationpredicted by this model and most hydropathy analyses. The model predicts the presence of a protonpathway within the N-terminal six-helix bundle of ProP (as opposed to the corresponding pathway foundwithin the C-terminal helix bundle of its paralogue, LacY). Replacement of residues within the N-terminalhelix bundle impaired the osmotic activation of ProP, providing the first indication that residues outsidethe C-terminal domain are involved in osmosensing. Some residues that were accessible from theperiplasmic side, as predicted by the structural model, were more susceptible to covalent labeling inpermeabilized membrane fractions than in intact bacteria. These residues may be accessible from thecytoplasmic side in structures not represented by our current model, or their limited exposure in vivo mayreflect constraints on transporter structure that are related to its osmosensory mechanism.