VAT-1 from Torpedo synaptic vesicles is a calcium binding protein: a study in bacterial expression systems

An essential role for phospholipase D in the recruitment of vesicle amine transport protein-1 to membranes in human neutrophils

Although phosphatidic acid (PA) regulates a wide variety of physiological processes, its targets remain poorly characterized in human neutrophils. By co-sedimentation with PA-containing vesicles we identified several PA-binding proteins including vesicle amine transport protein-1 (VAT-1), Annexin A3 (ANXA3), Rac2, Cdc42 and RhoG in neutrophil cytosol. Except for ANXA3, protein binding to PA-containing liposomes was calcium-independent. Cdc42 and RhoG preferentially interacted with PA whereas VAT-1 bound to PA or phosphatidylserine with the same affinity. VAT-1 translocated to neutrophil membranes upon N-formyl-methionyl-leucyl-phenylalanine (fMLF) stimulation. Inhibition of fMLF-induced PLD activity with the Src kinase inhibitor PP2, the selective inhibitor of PLD FIPI, or of PA formation with primary alcohols reduced VAT-1 translocation. In contrast, inhibition of PA hydrolysis with propranolol enhanced fMLF-mediated VAT-1 recruitment to membranes. PMA also redistributed VAT-1 to membranes in a PKC- and PLD-dependent manner. Though fMLF and PMA increased VAT-1 phosphorylation, different kinases appear to be involved. Cell fractionation revealed that a pool of VAT-1 was co-localized with primary, secondary and tertiary granules and plasma membrane markers in resting neutrophils. Stimulation with fMLF enhanced VAT-1 co-localization with CD32a, a plasma membrane marker. Confocal microscopy revealed that VAT-1 decorates granular structures at the cell periphery and double labeling with VAT-1/lactoferrin antibodies showed a partial co-localization with secondary granules in control and fMLF-stimulated cells. Characterization of these putative PA-binding proteins constitutes another step forward for a better understanding of the role of PLD-derived PA in neutrophil physiology.

Medium- and short-chain dehydrogenase/reductase gene and protein families : the MDR superfamily

The MDR superfamily with ~350-residue subunits contains the classical liver alcohol dehydrogenase (ADH), quinone reductase, leukotriene B4 dehydrogenase and many more forms. ADH is a dimeric zinc metalloprotein and occurs as five different classes in humans, resulting from gene duplications during vertebrate evolution, the first one traced to ~500 MYA (million years ago) from an ancestral formaldehyde dehydrogenase line. Like many duplications at that time, it correlates with enzymogenesis of new activities, contributing to conditions for emergence of vertebrate land life from osseous fish. The speed of changes correlates with function, as do differential evolutionary patterns in separate segments. Subsequent recognitions now define at least 40 human MDR members in the Uniprot database (corresponding to 25 genes when excluding close homologues), and in all species at least 10888 entries. Overall, variability is large, but like for many dehydrogenases, subdivided into constant and variable forms, corresponding to household and emerging enzyme activities, respectively. This review covers basic facts and describes eight large MDR families and nine smaller families. Combined, they have specific substrates in metabolic pathways, some with wide substrate specificity, and several with little known functions.

Proteomic analysis of early melanosomes: identification of novel melanosomal proteins

Melanin is a heterogeneous biopolymer produced only by specific cells termed melanocytes, which synthesize and deposit the pigment in specialized membrane-bound organelles known as melanosomes. Although melanosomes have been suspected of being closely related to lysosomes and platelets, the total number of melanosomal proteins is still unknown. Thus far, six melanosome-specific proteins have been identified, and the challenge is to characterize the complete proteome of the melanosome to further understand its mechanism of biogenesis. In this report, we used mass spectrometry and subcellular fractionation to identify protein components of early melanosomes. Using this approach, we have identified all 6 of the known melanosome-specific proteins, 56 proteins that are shared with other organelles, and confirmed the presence of 6 novel melanosomal proteins using western blotting and by immunohistochemistry.

The effect of calcium levels on synaptic proteins. A study on VAT-1 from Torpedo

In this study we compare major synaptic proteins from Torpedo electric organ to their homologues from mammalian brain. Most of these proteins are members of small gene families. We demonstrate a high degree of evolutionary conservation of most synaptic proteins. However, in the electric organ each gene family is represented only by a single member. We focus on VAT-1, a major protein of the vesicle membrane in Torpedo. VAT-1 is located on the synaptic vesicle membrane and is highly concentrated on the plasma membrane following the application of alpha-latrotoxin. Taking advantage of the relative simplicity of Torpedo synapses, we performed an in vitro study on the properties of VAT-1 affected by changes in Ca2+ levels. VAT-1 is a low affinity Ca2+ binding protein whose ability to bind Ca2+ resides mainly, but not entirely, on the carboxy-terminal domain of the protein. In the presence of Ca2+, the protein is organized in a high molecular mass complex, which is destabilized by depleting Ca2+. This effect occurs only by chelating Ca2+ ions, but not with other divalent ions. VAT-1 is not complexed to any of the proteins which were implicated in the docking/fusion complex such as VAMP, synaptophysin or syntaxin, regardless of Ca2+ levels. Dependence of the stability of protein complexes on Ca2+ levels is also demonstrated on Torpedo n-Sec1. The possible physiological implications of such Ca2+ dependence are discussed.

The synaptic vesicle and its targets

Synaptic vesicles play the central role in synaptic transmission. They are regarded as key organelles involved in synaptic functions such as uptake, storage and stimulus-dependent release of neurotransmitter. In the last few years our knowledge concerning the molecular components involved in the functioning of synaptic vesicles has grown impressively. Combined biochemical and molecular genetic approaches characterize many constituents of synaptic vesicles in molecular detail and contribute to an elaborate understanding of the organelle responsible for fast neuronal signalling. By studying synaptic vesicles from the electric organ of electric rays and from the mammalian cerebral cortex several proteins have been characterized as functional carriers of vesicle function, including proteins involved in the molecular cascade of exocytosis. The synaptic vesicle specific proteins, their presumptive function and targets of synaptic vesicle proteins will be discussed. This paper focuses on the small synaptic vesicles responsible for fast neuronal transmission. Comparing synaptic vesicles from the peripheral and central nervous systems strengthens the view of a high conservation in the overall composition of synaptic vesicles with a unique set of proteins attributed to this cellular compartment. Synaptic vesicle proteins belong to gene families encoding multiple isoforms present in subpopulations of neurons. The overall architecture of synaptic vesicle proteins is highly conserved during evolution and homologues of these proteins govern the constitutive secretion in yeast. Neurotoxins from different sources helped to identify target proteins of synaptic vesicles and to elucidate the molecular machinery of docking and fusion. Synaptic vesicle proteins and their markers are useful tools for the understanding of the complex life cycle of synaptic vesicles.