Tight binding of NAP-22 with acidic membrane lipids

Oligomerization and aggregation of NAP-22 with several metal ions

Metal ions participate in various biochemical processes such as electron transport chain, gene transcription, and enzymatic reactions. Furthermore, the aggregation promoting effect of several metal ions on neuronal proteins such as prion, tau, Aβ peptide, and α-synuclein, has been reported. NAP-22 (also called BASP1 or CAP-23) is a neuron-enriched calmodulin-binding protein and one of the major proteins in the detergent-resistant membrane microdomain fraction of the neuronal cell membrane. Previously, we showed oligomer formation of NAP-22 in the presence of several phospholipids and fatty acids. In this study, we found the aggregation of NAP-22 by FeCl2, FeCl3, and AlCl3 using native-PAGE. Oligomer or aggregate formation of NAP-22 by ZnCl2 or CuSO4 was shown with SDS-PAGE after cross-linking with glutaraldehyde. Morphological analysis with electron microscopy revealed the formation of large aggregates composed of small annular oligomers in the presence of FeCl3, AlCl3, or CuSO4. In case of FeCl2 or ZnCl2, instead of large aggregates, scattered annular and globular oligomers were observed. Interestingly, metal ion induced aggregation of NAP-22 was inhibited by several coenzymes such as NADP+, NADPH, or thiamine pyrophosphate. Since NAP-22 is highly expressed in the presynaptic region of the synapse, this result suggests the participation of metal ions not only on the protein and membrane dynamics at the presynaptic region, but also on the metabolic regulation though the interaction with coenzymes.

The effects of phospholipids and fatty acids on the oligomer formation of NAP-22

Recovery of various signal transduction molecules in the detergent-resistant membrane microdomain (DRM) fraction suggests the importance of this region in cellular functions. NAP-22 (also called BASP1 or CAP-23) is a neuron-enriched calmodulin-binding protein and one of the major proteins in the DRM fraction of the neuronal cell membrane. Previous studies showed tight binding activity of NAP-22 to acidic membrane lipids and the self-interaction of NAP-22, i.e., oligomerization. In this study, the effect of various phospholipids, lysophospholipids and fatty acids on the oligomerization of NAP-22 was studied through SDS-PAGE after chemical cross-linking and electron microscopic observation. High molecular mass oligomers were detected by SDS-PAGE after incubation in solutions containing over 20 mM NaCl at pH 6.5-8.5, even in the absence of lipid addition, and the addition of Ca²⁺/calmodulin abolished oligomerization. Higher molecular mass oligomer formation after incubation with acidic phospholipids was detected with gradient SDS-PAGE. Much higher mass oligomers were detected in the presence of polyunsaturated fatty acids. Electron microscopic analysis of the samples after SDS treatment showed tangled rope-like structures. Liposome-bound NAP-22 showed small oval or annular structures after cross-linking and SDS treatment. These oligomers were suggested to make the tangled rope-like structures, for annular structures of the same size were observed in the structure. These results suggest the participation of NAP-22 to liquid-liquid phase separation through oligomerization.

Interaction of dynamin I with NAP-22, a neuronal protein enriched in the presynaptic region

Neurons have well-developed membrane microdomains called "rafts" that are recovered as a detergent-resistant low-density membrane microdomain fraction (DRM). NAP-22 is one of the major protein components of neuronal DRM and localizes in the presynaptic region. In order to know the role of NAP-22 in the synaptic transmission, NAP-22 binding proteins in the cytosol were searched with an affinity screening with NAP-22 as a bait and several protein bands were detected. Using mass-analysis and western blotting, one of the main band of ∼90 kDa was identified as dynamin I. The GTPase activity of dynamin I was partly inhibited by NAP-22 expressed in bacteria and this inhibition was recovered by the addition of calmodulin, a NAP-22 binding protein. The GTPase activity of dynamin was known to be activated with acidic membrane lipids such as phosphatidylserine and the addition of NAP-22, a phosphatidylserine binding protein, inhibited the activation of the GTPase by this lipid. Since NAP-22 localizes on the presynaptic plasma membrane and on synaptic vesicles, these results suggest the participation of NAP-22 in the membrane cycling through binding to dynamin and acidic membrane lipids at the presynaptic region.

Identifying key membrane protein lipid interactions using mass spectrometry

With the recent success in determining membrane protein structures, further detailed understanding of the identity and function of the bound lipidome is essential. Using an approach that combines high-energy native mass spectrometry (HE-nMS) and solution-phase lipid profiling, this protocol can be used to determine the identity of the endogenous lipids that directly interact with a protein. Furthermore, this method can identify systems in which such lipid binding has a major role in regulating the oligomeric assembly of membrane proteins. The protocol begins with recording of the native mass spectrum of the protein of interest, under successive delipidation conditions, to determine whether delipidation leads to disruption of the oligomeric state. Subsequently, we propose using a bipronged strategy: first, an HE-nMS platform is used that allows dissociation of the detergent micelle at the front end of the instrument. This allows for isolation of the protein-lipid complex at the quadrupole and successive fragmentation at the collision cell, which leads to identification of the bound lipid masses. Next, simultaneous coupling of this with in-solution LC-MS/MS-based identification of extracted lipids reveals the complete identity of the interacting lipidome that copurifies with the proteins. Assimilation of the results of these two sets of experiments divulges the complete identity of the set of lipids that directly interact with the membrane protein of interest, and can further delineate its role in maintaining the oligomeric state of the protein. The entire procedure takes 2 d to complete.

A proteomic analysis of contextual fear conditioned rats reveals dynamic modifications in neuron and oligodendrocyte protein expression in the dentate gyrus

Contextual memory is an intricate process involving synaptic plasticity and network rearrangement. Both are governed by many molecular processes including phosphorylation and modulation of protein expression. However, little is known about the molecules involved in it. Here, we exploited the advantages of a quantitative proteomic approach to identify a great number of molecules in the rat dentate gyrus after a contextual fear conditioning session. Our results allowed us to highlight protein expression patterns, not only related to neuroplasticity, but also to myelin structure, such as myelin basic protein and myelin proteolipid protein showing a decrease in expression. Validation of the modification in protein expression reveals a dynamic profile during the 48 h following the fear conditioning session. The expression of proteins involved in neurite outgrowth, such as BASP-1 and calcineurin B1, and in synaptic structure and function, VAMP2 and RAB3C, was increased in the dentate gyrus of rats submitted to fear conditioning compared to controls. We showed that the increase in BASP-1 protein was specific to fear conditioning learning as it was not present in immediate-shock rats, neither in rats exposed to a novel environment without being shocked. As myelin is known to stabilise synaptic network, the decrease in myelin proteins suggests a neuroglia interactive process taking place in the dentate gyrus in the 24 h following contextual fear learning, which has never been demonstrated before. These results therefore open the way to the study of new plasticity mechanisms underlying learning and memory.