neither the lipid targets for this protein nor the residues responsible for binding are well characterized. Our study attempts to identify the respective roles of (a) phosphoinositides such as phosphatidylinositol (4,5)-bisphosphate (PIP2) and (b) anionic backgroundlipids such as phosphatidylserine (PS) in binding of the granuphilin C2A domain to liposomes. Affinities are measured using protein-to-membrane FRET and titration with the competitive inhibitor inositol (1,2,3,4,5,6)-hexakisphosphate (IP6). Granuphilin C2A binds to liposomes containing phosphatidylcholine, dansyl-phosphatidylethanolamine, and PS and/or PIP2. Decreased FRET is measured as the protein is displaced from lipid upon titration with IP6. Granuphilin C2A has similar affinity for liposomes containing either 24% PS or 2% PIP2, but affinity increases~100-fold in the presence of both target lipids. Binding site(s) for the two lipids are being probed using site-directed mutagenesis and NMR. Overall, granuphilin demonstrates the capability to bind to background anionic lipids and phosphoinositides independently, but the presence of both lipids greatly increases the binding affinity of this protein. This suggests that granuphilin may serve as a coincidence detector to target vesicles to sites of secretion on the plasma membrane.
Neurofilaments (NFs) are essential building blocks of axonal architecture. Abnormal behavior of these cytostructural elements has been associated with several neuromuscular disorders such as Amyotropic Lateral Sclerosis (ALS). NFs are assembled from three subunits: Low (NFL), Medium (NFM) and Heavy (NFH). These subunits are characterized by a common alpha helical rod domain and carboxyl terminal domains of different lengths specific to each subunit. The tails project from the core of the filament and contain a number of KSP repeat motifs that belongs to the sites for phosphorylation. Especially, the C-terminal tails of NFM and NFH that have relatively longer lengths and higher number of KSP repeats were found to be the key participants of the sidearmmediated interfilament interactions that regulate the axonal diameter. Though it has been established that that the sidearms play a key functional role, little is known about the roles of individual subunits and the effect of phosphorylation on their behavior. Initially, it was believed that the NFH sidearms play a more integral role in determining axonal structure due to the presence of longer polypeptides and relatively higher KSP repeat units. However, recent studies showed that deleting NFH from neurofilaments does not affect axonal diameter, suggesting that NFM may in fact be the key player. In view of this, it is essential to have an understanding of the morphological behavior of the NFM sidearm in response to physiological conditions. In the present study we carried out MD simulations of human and mouse NFM C terminals under different phosphorylation and ionic conditions. The results from these studies provide useful molecular level insight into the structural changes of NFM sidearms in response to phosphorylation, ionic concentrations. The present study reveals sidearm-mediated regulation mechanism of axonal caliber.
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