The topology of helical membrane proteins is generally defined during insertion of the transmembrane helices, yet it is now clear that it is possible for topology to change under unusual circumstances. It remains unclear, however, if topology reorientation is part of normal biogenesis. For dual topology dimer proteins such as the multidrug transporter EmrE, there may be evolutionary pressure to allow topology flipping so that the populations of both orientations can be equalized. We previously demonstrated that when EmrE is forced to insert in a distorted topology, topology flipping of the first transmembrane helix can occur during translation. Here, we show that topological malleability also extends to the C-terminal helix and that even complete topology inversion of the entire EmrE protein can occur after the full protein is translated and inserted. Thus, topology rearrangements are possible during normal biogenesis. Wholesale topology flipping is remarkable given the physical constraints of the membrane and expands the range of possible membrane protein folding pathways, both productive and detrimental.Keywords: membrane topology; topology flipping; membrane protein folding; topology inversion; topology change; transmembrane helix Summary Once transmembrane segments are inserted in a particular topology, the bilayer would appear to present a severe challenge for subsequent topology changes. Nevertheless, topology changes have been documented when the protein or membrane is distorted. Here, we find that full topological inversion of a membrane protein consisting of four transmembrane helices can occur under normal physiological conditions.The lipid bilayer presents an apolar barrier to the transport of polar molecules. Even the simple process of lipid flip-flop, moving the lipid headgroup from one bilayer leaflet to another, is associated
Abstract-Owing to recent interest in sensor networks for military and security applications, studies of the security vulnerabilities of these networks are becoming increasingly important. In the present paper, the problem of estimating the topology of an adversarial sensor network is considered. The adversarial network is assumed to employ strong encryption, so that its transmitted packets are assumed to be unreadable by the observer. Thus, the algorithms are required to make use of the time correlations in channel uses by the adversarial network. Assuming the use of the MACA protocol, our algorithms are capable of estimating both the routes used by nodes in the adversarial sensor network, and as the identities of the nodes that are within each other's neighborhood (i.e., within radio range of each other), so that an attack could be designed for maximum effect. Results are presented which show that route estimation can be accomplished quickly using our algorithm, and that neighborhood estimation can be accomplished in a reasonable amount of time.
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