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SUMMARY Cells maintain integrity despite changes in their mechanical properties elicited during growth and environmental stress. How cells sense their physical state and compensate for cell-wall damage is poorly understood, particularly in plants. Here we report that FERONIA (FER), a plasma-membrane-localized receptor kinase from Arabidopsis, is necessary for the recovery of root growth after exposure to high salinity, a widespread soil stress. The extracellular domain of FER displays tandem regions of homology with malectin, an animal protein known to bind diglucose in vitro and important for protein quality control in the endoplasmic reticulum. The presence of malectin-like domains in FER and related receptor kinases has led to widespread speculation that they interact with cell-wall polysaccharides and can potentially serve a wall-sensing function. Results reported here show that salinity causes softening of the cell wall and that FER is necessary to sense these defects. When this function is disrupted in the fer mutant, root cells explode dramatically during growth recovery. Similar defects are observed in the mur1 mutant, which disrupts pectin cross-linking. Furthermore, fer cell-wall integrity defects can be rescued by treatment with calcium and borate, which also facilitate pectin cross-linking. Sensing of these salinity-induced wall defects might therefore be a direct consequence of physical interaction between the extracellular domain of FER and pectin. FER-dependent signaling elicits cell-specific calcium transients that maintain cell-wall integrity during salt stress. These results reveal a novel extracellular toxicity of salinity, and identify FER as a sensor of damage to the pectin-associated wall.

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The redox-regulated open/closed conformational switch of hPDI endows the protein with versatile target-binding capacities for its enzymatic and chaperone functions.

The major facilitator superfamily (MFS) is the largest family of secondary active transporters and is present in all life kingdoms. Detailed structural basis of the substrate transport and energycoupling mechanisms of these proteins remain to be elucidated. YajR is a putative proton-driven MFS transporter found in many Gram-negative bacteria. Here we report the crystal structure of Escherichia coli YajR at 3.15 Å resolution in an outward-facing conformation. In addition to having the 12 canonical transmembrane helices, the YajR structure includes a unique 65-residue C-terminal domain which is independently stable. The structure is unique in illustrating the functional role of "sequence motif A." This highly conserved element is seen to stabilize the outward conformation of YajR and suggests a general mechanism for the conformational change between the inward and outward states of the MFS transporters.T ransporters are a type of membrane protein essential for all living cells that actively up-take nutrition and export metabolic substances and toxic materials across cellular membranes. Transporters are divided into two major types based on their energy sources. Although primary active transporters directly consume energy from ATP hydrolysis to drive substrate transport, secondary active transporters use energy derived from the electrochemical potential across the cell membrane. The major facilitator superfamily (MFS) is the largest class of secondary transporters and is present in all life kingdoms (1). For example, 25% of prokaryotic transporters belong to the MFS family (2), and the human genome contains over 110 MFS proteins (3). Currently, 3D crystal structures of nine bacterial MFS transporters (4-13) and one from fungi (9) have been reported at medium-high resolution. These studies have shown that MFS proteins contain a 12-transmembrane (TM) helix core composed of two six-helix rigid domains forming a central TM channel, which transports substrates using a rocker-switch mechanism (5). In such a mechanism, MFS proteins are believed to switch between two major conformations, inward and outward, which differ by an ∼40°rotation of one domain relative to the other. Both conformations have been captured in MFS crystal structures. However, many questions remain to be addressed, particularly those related to energy coupling and functional roles of conserved motifs.YajR, a 49-kDa transporter of the MFS family, has putatively been classified as a drug efflux protein solely on the basis of amino acid sequence analysis (14). In Escherichia coli, YajR consists of 454 amino acid residues. Besides containing 12 TM helices, YajR is predicted to possess an extra domain of about 65 residues at the C-terminal. Of the MFS proteins with reported 3D structures, the TM core of YajR shares highest sequence homology (21% identity) with EmrD (SI Appendix, Fig. S1A), which belongs to the 12-TM drug-resistance H + -driven antiporter (DHA12) subfamily (15). The YajR gene is found in a number of Gram-negative bacteria, and it shows high ...

SUMMARY The location, size and number of synapses critically influence the specificity and strength of neural connections. In axons, synaptic vesicle (SV) and active zone (AZ) proteins are transported by molecular motors and accumulate at discrete presynaptic loci. Little is known about the mechanisms coordinating presynaptic protein transport and deposition to achieve proper distribution of synaptic material. Here we show that SV and AZ proteins exhibit extensive co-transport and undergo frequent pauses. At the axonal and synaptic pause sites, the balance between the capture and dissociation of mobile transport packets determines the extent of presynaptic assembly. The small G protein ARL-8 inhibits assembly by promoting dissociation, while a JNK kinase pathway and AZ assembly proteins inhibit dissociation. Furthermore, ARL-8 directly binds to the UNC-104/KIF1A motor to limit the capture efficiency. Together, molecular regulation of the dichotomy between axonal trafficking and local assembly controls vital aspects of synapse formation and maintenance.

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Multiple PDZ domain scaffold protein Par-3 and phosphoinositides (PIPs) are required for polarity in diverse cell types. We show that the second PDZ domain of Par-3 binds to phosphatidylinositol (PI) lipid membranes with high affinity. We further demonstrate that a large subset of PDZ domains in mammalian genomes are capable of binding to PI lipid membranes, indicating that lipid binding is the second most prevalent interaction mode of PDZ domains known to date. The biochemical and structural basis of Par-3 PDZ2-mediated membrane interaction is characterized in detail. The membrane binding capacity of Par-3 PDZ2 is critical for epithelial cell polarization. Interestingly, the lipid phosphatase PTEN directly binds to the third PDZ domain of Par-3. The concatenation of the PIP-binding PDZ2 and the lipid phosphatase PTEN-binding PDZ3 endows Par-3 as an ideal scaffold protein for integrating PIP signaling events during cellular polarization.