Widely used higher-dimensional Fourier transform (FT) NMR spectroscopy suffers from two major drawbacks: (i) The minimal measurement time of an N-dimensional FT NMR experiment, which is constrained by the need to sample N - 1 indirect dimensions, may exceed by far the measurement time required to achieve sufficient signal-to-noise ratios. (ii) The low resolution in the indirect dimensions severely limits the precision of the indirect chemical shift measurements. To relax on constraints arising from these drawbacks, we present here an acquisition scheme which is based on the phase-sensitive joint sampling of the indirect dimensions spanning a subspace of a conventional NMR experiment. This allows one to very rapidly obtain high-dimensional NMR spectral information. Because the phase-sensitive joint sampling yields subspectra containing "chemical shift multiplets", alternative data processing is required for editing the components of the multiplets. The subspectra are linearly combined using a so-called "G-matrix" and subsequently Fourier-transformed. The chemical shifts are multiply encoded in the resonance lines constituting the shift multiplets. This corresponds to performing statistically independent multiple measurements, and the chemical shifts can thus be obtained with high precision. To indicate that a combined G-matrix and FT is employed, we named the new approach "GFT NMR spectroscopy". GFT NMR opens new avenues to establish high-throughput protein structure determination, to investigate systems with a higher degree of chemical shift degeneracy, and to study dynamic phenomena such as slow folding of biological macromolecules in greater detail.

Synaptic cell adhesion molecules (CAMs) regulate synapse formation through their trans-synaptic and heterophilic adhesion. Here we show that postsynaptic netrin-G ligand (NGL) CAMs associate with netrin-G CAMs in an isoform-specific manner and, through their cytosolic tail, with the abundant postsynaptic scaffold postsynaptic density-95 (PSD-95). Overexpression of NGL-2 in cultured rat neurons increased the number of PSD-95-positive dendritic protrusions. NGL-2 located on heterologous cells or beads induced functional presynaptic differentiation in contacting neurites. Direct aggregation of NGL-2 on the surface membrane of dendrites induced the clustering of excitatory postsynaptic proteins. Competitive inhibition by soluble NGL-2 reduced the number of excitatory synapses. NGL-2 knockdown reduced excitatory, but not inhibitory, synapse numbers and currents. These results suggest that NGL regulates the formation of excitatory synapses.

Synaptic adhesion molecules regulate multiple steps of synapse formation and maturation. The great diversity of neuronal synapses predicts the presence of a large number of adhesion molecules that control synapse formation through trans-synaptic and heterophilic adhesion. We identified a previously unknown trans-synaptic interaction between netrin-G ligand-3 (NGL-3), a postsynaptic density (PSD) 95-interacting postsynaptic adhesion molecule, and leukocyte common antigen-related (LAR), a receptor protein tyrosine phosphatase. NGL-3 and LAR expressed in heterologous cells induced pre- and postsynaptic differentiation in contacting axons and dendrites of cocultured rat hippocampal neurons, respectively. Neuronal overexpression of NGL-3 increased presynaptic contacts on dendrites of transfected neurons. Direct aggregation of NGL-3 on dendrites induced coclustering of excitatory postsynaptic proteins. Knockdown of NGL-3 reduced the number and function of excitatory synapses. Competitive inhibition by soluble LAR reduced NGL-3-induced presynaptic differentiation. These results suggest that the trans-synaptic adhesion between NGL-3 and LAR regulates excitatory synapse formation in a bidirectional manner.

Synaptic cell adhesion molecules (CAMs) are known to play key roles in various aspects of synaptic structures and functions, including early differentiation, maintenance, and plasticity. We herein report the identification of a family of cell adhesion-like molecules termed SALM that interacts with the abundant postsynaptic density (PSD) protein PSD-95. SALM2, a SALM isoform, distributes to excitatory, but not inhibitory, synaptic sites. Overexpression of SALM2 increases the number of excitatory synapses and dendritic spines. Mislocalized expression of SALM2 disrupts excitatory synapses and dendritic spines. Bead-induced direct aggregation of SALM2 results in coclustering of PSD-95 and other postsynaptic proteins, including GKAP and AMPA receptors. Knockdown of SALM2 by RNA interference reduces the number of excitatory synapses and dendritic spines and the frequency, but not amplitude, of miniature excitatory postsynaptic currents. These results suggest that SALM2 is an important regulator of the differentiation of excitatory synapses.

Liprin-␣/SYD-2 is a family of multidomain proteins with four known isoforms. One of the reported functions of liprin-␣ is to regulate the development of presynaptic active zones, but the underlying mechanism is poorly understood. Here we report that liprin-␣ directly interacts with the ERC (ELKS-Rab6-interacting protein-CAST) family of proteins, members of which are known to bind RIMs, the active zone proteins that regulate neurotransmitter release. In vitro results indicate that ERC2/CAST, an active zone-specific isoform, interacts with all of the known isoforms of liprin-␣ and that liprin-␣1 associates with both ERC2 and ERC1b, a splice variant of ERC1 that distributes to both cytosolic and active zone regions. ERC2 colocalizes with liprin-␣1 in cultured neurons and forms a complex with liprin-␣1 in brain. Liprin-␣1, when expressed alone in cultured neurons, shows a partial synaptic localization. When coexpressed with ERC2, however, liprin-␣1 is redistributed to synaptic sites. Moreover, roughly the first half of ERC2, which contains the liprin-␣-binding region, is sufficient for the synaptic localization of liprin-␣1 while the second half is not. These results suggest that the interaction between ERC2 and liprin-␣ may be involved in the presynaptic localization of liprin-␣ and the molecular organization of presynaptic active zones.