SYN1 | |||||||||||||||||||||||||||||||||||||||||||||||||||
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Aliases | SYN1 , SYN1a, SYN1b, SYNI, Synapsin I, MRX50, EPILX | ||||||||||||||||||||||||||||||||||||||||||||||||||
External IDs | OMIM: 313440 MGI: 98460 HomoloGene: 48483 GeneCards: SYN1 | ||||||||||||||||||||||||||||||||||||||||||||||||||
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Synapsin I, is the collective name for Synapsin Ia and Synapsin Ib, two nearly identical phosphoproteins that in humans are encoded by the SYN1 gene. [5] [6] In its phosphorylated form, Synapsin I may also be referred to as phosphosynaspin I. Synapsin I is the first of the proteins in the synapsin family of phosphoproteins in the synaptic vesicles present in the central and peripheral nervous systems. Synapsin Ia and Ib are close in length and almost the same in make up, however, Synapsin Ib stops short of the last segment of the C-terminal in the amino acid sequence found in Synapsin Ia.
The synapsin I protein is a member of the synapsin family that are neuronal phosphoproteins which associate with the cytoplasmic surface of synaptic vesicles. Family members are characterized by common protein domains, and they are implicated in synaptogenesis and the modulation of neurotransmitter release, suggesting a potential role in several neuropsychiatric diseases.
The phosphoprotein plays a role in regulation of axonogenesis and synaptogenesis. The protein serves as a substrate for several different protein kinases and phosphorylation may function in the regulation of this protein in the nerve terminal. [6]
Synapsin I is found in two isoforms of the protein, Synapsin Ia and Synapsin Ib, with Synapsin Ib being a slightly shorter version of the protein. Both Synapsin I proteins are highly basic with a pI in the range of 10.3 and 10.2, respectively. Both isoforms are phosphorylated at identical locations within their protein sequences at the same three serine residues.
Synapsin I phosphoproteins make up approximately 6% of the total protein in synaptic vesicles. [7] Among bovine, rat, and human it has been shown to be 95% homologous, with the central 'C' domain evolutionarily conserved. This phosphoprotein is loosely associated with the vesicular membrance and is easily dissociated by treatment with a salt, versus a detergent being required for its removal from the membrane.
Synapsin I proteins are made up of a globular portion at the N-terminal and an elongated C-terminal domain, rendering them largely elongated. Synapsin Ib has the same protein domains as synapsin Ia, however synapsin Ib lacks the last C-terminal segment, making it slightly shorter in its elongated domain. 706 amino acids comprise synapsin Ia, and starting from the N-terminal, the same first 670 amino acids comprise synapsin Ib.
Rich in the amino acids proline and glycine, the compositional and structural natures of this protein are somewhat similar to collagen. This aided in the early determination of its structure using collagenase, which was later confirmed by amino acid sequencing and modern techniques. Cleavage of synapsin I by collagenase fragments the elongated C-terminal and leaves the globular N-terminal domain intact. [8]
Amino acid sequencing has shown that synapsin I has common N-terminals across both isoforms and shares the same N-terminal as synapsin II. Synapsin I isoforms differ from synapsin II isoforms in their C-terminal domains as well. [9] Further research has been done on the interactions of synapsin I, synapsin II, and synapsin III with each other to create heterodimers of the proteins in COS cells. [10]
Synapsin I is present in the nerve terminal of axons, specifically in the membranes of synaptic vesicles based on immunocytochemistry. [11] This phosphoprotein is as an endogenous substrate bound to the vesicular membrane. It is phosphorylated by four known classes of protein kinases including those activated by cAMP, [12] [13] calcium/calmodulin, [14] mitogen, and cyclin. Both isoforms have the same six phosphorylation sites:
The N-terminal globular domain contains three sites: the cAMP-dependent protein kinase-mediated phosphorylation site near the end in domain A, and two sites further in, in domain B, mediated by mitogen-activated protein kinase (MAP kinase). The tail portion of the protein, the C-terminal end, bears three phosphorylation sites: two sites at which calcium/calmodulin dependent protein kinase II acts, and a third site at which MAP kinase and cyclin-dependent protein kinase (CDK) acts. Specificity for calcium/calmodulin dependent protein kinase binding to Synapsin I is very high in comparison to other substrate proteins. [15] Cyclic AMP-dependent protein kinase is unique in its mechanism of activation. The protein kinase is composed of two regulatory (R) subunits and two catalytic (C) subunits, creating a tetrameric holoenzyme. Cyclic AMP binds to the regulatory subunits of cAMP-dependent protein kinase and causes the dissociation of its regulatory subunits from the catalytic subunits, generating the active form of the kinase. This active form of the protein kinase catalyses the phosphorylation of Synapsin I. The phosphorylated form of Synapsin I is referred to as phosphosynapsin I.
Depolarization of the presynaptic membrane induces a calcium ion influx into the axonal nerve terminal of neurons, and increases the intracellular concentration of calcium ions. Synapsin I was shown to be phosphorylated by this calcium influx. [16] The calcium ion, Ca2+, binds to calmodulin to form a calcium/calmodulin complex which then activates the calcium/calmodulin-dependent protein kinase, in turn triggering phosphorylation. [14] Calcium/calmodulin-dependent phosphorylation of synapsin I causes dissociation of synapsin I from the vesicular membrane.
In the nerve terminal ending, there are two pools of synaptic vesicles, the reserve pool and the ready-release pool. The reserve pool refers to the synaptic vesicles that are not ready to release neurotransmitters and the ready-release pool refers to the vesicles which are primed to release their neurotransmitters across the presynaptic cytoplasmic membrane and into the synaptic cleft. The removal of Synapsin I from synaptic vesicles is thought to mobilize synaptic vesicles from the reserve pool to the release-ready pool, thereby modulating neurotransmitter release. Since it is only present in the vesicles in the reserve pool, the non-phosphorylated form of Synapsin I is considered to be an inhibitory regulator of neurotransmission.
The synapsin I protein has been shown to interact with NOS1AP [17] and SYN2. [10]
Mutations in the SYN1 gene may be associated with X-linked disorders with primary neuronal degeneration such as Rett syndrome. [6]
The original member of the synapsin family, Synapsin I, is the first neuronal synaptic membrane protein that was initially observed in 1973, by Tetsufumi Ueda in the laboratory of Nobel Prize winner Paul Greengard. [18] Originally named Protein I, Synapsin I, it was detected through radioactive phosphorus (P-32) incorporated through protein phosphorylation, catalyzed by a cyclic AMP-dependent protein kinase (enzyme) naturally found in the neuronal membrane at the synaptic cleft in rats. In 1977, this first synaptic phosphoprotein was also purified and first characterized by Tetsufumi Ueda at the same laboratory at Yale University under Paul Greengard.
The novel techniques used to discover Synapsin I, were a combination of SDS gel electrophoresis and autoradiography developed by Tetsufumi Ueda in Greengard’s laboratory, that significantly enhanced the way proteins activated by phosphorylation could be observed. More specifically, this was accomplished by autoradiography measuring the radioactivity of the individual protein bands phosphorylated by radioactive adenosine triphosphate. Hiroo Maeno, a lab colleague, assisted with the sample preparations and radio-labelling ATP with P-32 at the gamma phosphate. [18]
The discovery of the synaptic membrane protein and the methodology by which it was discovered, are considered both groundbreaking advancements in the analysis of phosphorylated proteins, and introduced the identification of specific proteins.
Synapsin I is also the first collagenous protein to be described in the nervous system. [13]
The synapsins are a family of proteins that have long been implicated in the regulation of neurotransmitter release at synapses. Specifically, they are thought to be involved in regulating the number of synaptic vesicles available for release via exocytosis at any one time. Synapsins are present in invertebrates and vertebrates and are strongly conserved across all species. They are expressed in highest concentration in the nervous system, although they also express in other body systems such as the reproductive organs, including both eggs and spermatozoa. Synapsin function also increases as the organism matures, reaching its peak at sexual maturity.
Telokin is an abundant protein found in smooth-muscle. It is identical to the C-terminus of myosin light-chain kinase. Telokin may play a role in the stabilization of unphosphorylated smooth-muscle myosin filaments. Because of its origin as the C-terminal end of smooth muscle myosin light chain kinase, it is called "telokin".
Ca2+
/calmodulin-dependent protein kinase II is a serine/threonine-specific protein kinase that is regulated by the Ca2+
/calmodulin complex. CaMKII is involved in many signaling cascades and is thought to be an important mediator of learning and memory. CaMKII is also necessary for Ca2+
homeostasis and reuptake in cardiomyocytes, chloride transport in epithelia, positive T-cell selection, and CD8 T-cell activation.
Phosphorylase kinase (PhK) is a serine/threonine-specific protein kinase which activates glycogen phosphorylase to release glucose-1-phosphate from glycogen. PhK phosphorylates glycogen phosphorylase at two serine residues, triggering a conformational shift which favors the more active glycogen phosphorylase “a” form over the less active glycogen phosphorylase b.
Growth Associated Protein 43 (GAP43) is a protein encoded by the GAP43 gene in humans.
Synaptojanin is a protein involved in vesicle uncoating in neurons. This is an important regulatory lipid phosphatase. It dephosphorylates the D-5 position phosphate from phosphatidylinositol (3,4,5)-trisphosphate (PIP3) and Phosphatidylinositol (4,5)-bisphosphate(PIP2). It belongs to family of 5-phosphatases, which are structurally unrelated to D-3 inositol phosphatases like PTEN. Other members of the family of 5'phosphoinositide phosphatases include OCRL, SHIP1, SHIP2, INPP5J, INPP5E, INPP5B, INPP5A and SKIP.
Calcium/calmodulin-dependent protein kinase type II subunit alpha (CAMKIIα), a.k.a.Ca2+/calmodulin-dependent protein kinase II alpha, is one subunit of CamKII, a protein kinase (i.e., an enzyme which phosphorylates proteins) that in humans is encoded by the CAMK2A gene.
Calcium/calmodulin-dependent protein kinase type II beta chain is an enzyme that in humans is encoded by the CAMK2B gene.
Calcium/calmodulin-dependent protein kinase type IV is an enzyme that in humans is encoded by the CAMK4 gene.
Calcium/calmodulin-dependent protein kinase type II gamma chain is an enzyme that in humans is encoded by the CAMK2G gene.
In enzymology, a ceramide kinase, also abbreviated as CERK, is an enzyme that catalyzes the chemical reaction:
In enzymology, an elongation factor 2 kinase is an enzyme that catalyzes the chemical reaction:
Plasma membrane calcium-transporting ATPase 4 is an enzyme that in humans is encoded by the ATP2B4 gene.
Calcium/calmodulin-dependent protein kinase type 1 is an enzyme that in humans is encoded by the CAMK1 gene.
Calcium/calmodulin-dependent protein kinase type II delta chain is an enzyme that in humans is encoded by the CAMK2D gene.
Phosphorylase b kinase gamma catalytic chain, skeletal muscle isoform is an enzyme that in humans is encoded by the PHKG1 gene.
Mitogen-activated protein kinase 12 is an enzyme that in humans is encoded by the MAP3K12 gene.
Synapsin II is the collective name for synapsin IIa and synapsin IIb, two nearly identical phosphoproteins in the synapsin family that in humans are encoded by the SYN2 gene. Synapsins associate as endogenous substrates to the surface of synaptic vesicles and act as key modulators in neurotransmitter release across the presynaptic membrane of axonal neurons in the nervous system.
The active zone or synaptic active zone is a term first used by Couteaux and Pecot-Dechavassinein in 1970 to define the site of neurotransmitter release. Two neurons make near contact through structures called synapses allowing them to communicate with each other. As shown in the adjacent diagram, a synapse consists of the presynaptic bouton of one neuron which stores vesicles containing neurotransmitter, and a second, postsynaptic neuron which bears receptors for the neurotransmitter, together with a gap between the two called the synaptic cleft. When an action potential reaches the presynaptic bouton, the contents of the vesicles are released into the synaptic cleft and the released neurotransmitter travels across the cleft to the postsynaptic neuron and activates the receptors on the postsynaptic membrane.
Mary Bernadette Kennedy is an American biochemist and neuroscientist. She is a member of the American Academy of Arts and Sciences, and is the Allen and Lenabelle Davis Professor of Biology at the California Institute of Technology, where she has been a member of the faculty since 1981. Her research focuses on the molecular mechanisms of synaptic plasticity, the process underlying formation of memory in the central nervous system. Her lab uses biochemical and molecular biological methods to study the protein machinery within a structure called the postsynaptic density. Kennedy has published over 100 papers with over 20,000 total citations.
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