Synapsin I

Key takeaways

Synapsin I belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Synapsin I to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Synapsin I from memory before moving on to harder problems.

Reference excerpt

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. 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 makeup; however, Synapsin Ib stops short of the last segment of the C-terminal in the amino acid sequence found in Synapsin Ia.

Protein 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. 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. 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.

Structure 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. 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. 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.

Function Synapsin I is present in the nerve terminal of axons, specifically in the membranes of synaptic vesicles based on immunocytochemistry. 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, calcium/calmodulin, 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. 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. 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. 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.

Interactions The synapsin I protein has been shown to interact with NOS1AP and SYN2.

Clinical significance Mutations in the SYN1 gene may be associated with X-linked disorders with primary neuronal degeneration such as Rett syndrome.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Synapsin I

Start with the simplest possible case. Write down what Synapsin I claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Synapsin I before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Synapsin I ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Synapsin I

In research: Synapsin I appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Synapsin I in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Synapsin I is common in secondary-school and first-year university syllabi. It links to neighbouring topics Genes on human chromosome X, Human proteins, Molecular neuroscience, so understanding it makes those chapters shorter.
In everyday life: Look for Synapsin I outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Synapsin I in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Synapsin I means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Synapsin I out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Synapsin I in simple terms?

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. In its phosphorylated form, Synapsin I may also be referred to as phosphosynaspin I.

Why does Synapsin I matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Synapsin I?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Synapsin I.