1. Introduction

The synucleins are a family of natively unfolded (or intrinsically unstructured) proteins consisting of α-, β-, and γ-synuclein involved in neurodegenerative diseases and cancer ^[1]. The current number of publications on synucleins has exceeded 16,000 and they remain the subject of constant interest for over 35 years. The two reasons explain this unchangeable attention. First, synuclein's association with several severe human diseases ^[2][3]. Second, the lack of understanding of the functional roles under normal physiological conditions. Traditional areas of peak research interest which still remain high among last year's publications are comparative studies of structural features as well as functional research on of three members of the synuclein family. Another popular research topic in the area are mechanisms controlling α-synuclein accumulation, aggregation, and fibrillation. Exciting fast-growing field of more recent research is α-synuclein and epigenetics.

2. Three Members of the Synuclein Family

Synucleins are a family of small prone to aggregate intrinsically disordered proteins (IDP). Since synucleins are involved in severe human diseases, understandably most research is directed at unveiling their role in pathology. As a result, we know much more about their contribution to pathological mechanisms than their normal functions, which are still not fully understood.

The first synuclein was identified using an antiserum against purified cholinergic synaptic vesicles as a 143 amino acid presynaptic protein in the electric organ of Torpedo californica ^[4]. Later, two additional isoforms belonging to the synuclein family were identified ^[5]. Three evolutionary conserved members of this family are highly expressed in the vertebrate nervous system and have been found in all vertebrates ^[6]. Importantly, no counterparts of synucleins were identified in invertebrates, indicating that they are vertebrate-specific proteins. Further analysis demonstrates that the number of synuclein members may differ among vertebrates. While three genes encoding α-, β-, and γ-synuclein are present in mammals and birds, a variable number of synuclein genes and corresponding proteins are expressed in fish, depending on the species. For example, four synuclein genes are identified in fugu, encoding for α, β, and two g (g1 and g2) isoforms, but three genes are detected in zebrafish ^[7][8].

IDPs constitute about one-third of the human proteome ^[9] pointing to the importance of their structural organization. The absence of a defined structure confers members of the synuclein family conformational flexibility, allowing them to participate in dynamic and transient molecular interactions.

The overwhelming majority of publications on synucleins are dedicated to α-synuclein due to its involvement in Parkinson’s disease (PD) and other synucleinopathies ^[10]. Yet, two other proteins β- and γ- synucleins are members of the same family concentrated in neuronal terminals and implicated in the long-term regulation and maintenance of nerve terminal function and dopamine homeostasis ^[11]. As a result, they deserve more attention to better understand the functionality of nervous system and role in carcinogenesis.

The members of the synuclein family generally share a similar N-terminal region of which α-synuclein has been supposed to form a single or two distinct α-helices. This transition is induced when α-synuclein interacts with the lipid bilayer of membranes. A C-terminal region contains negatively charged residues and prolines. The C-terminal domain differs in amino acid sequence among synucleins and regulates their solubility depending on its length and charge. Amino acid positions 61–95 in α-synuclein are called NAC, and amino acids 71–82 within the α-synuclein NAC region have been proposed to be critical for β-sheet-rich aggregation in vitro ^[12][13].

3. Common Structure of Members of the Synuclein Family

All three synucleins have a substantial degree of sequence and structural similarities: VTGVTAVAQKTV in α-synuclein, FSGAGNIAAATG in β-synuclein, and VSSVNTVATKTV in γ-synuclein. Amino acids belonging to the 71–82 stretch are necessary and sufficient for α-synuclein fibrillization ^[13].

Furthermore, the substitutions of three defined amino acids (E35K + E46K + E61K = '3K') are sufficient to render all synucleins significantly more toxic than their wild-type counterparts ^[13]. These substitutions increase α-synuclein- membrane interactions pointing to a membrane-associated mechanism of synuclein toxicity. Binding to lipids converts the N-terminal domain to an α-helical structure. The N-terminal helicity negatively correlates with aggregation potential, while the C-termini differ among synucleins regulating their solubility.

Newberry et al. ^[14] used deep mutational scanning to reveal the structural basis for α-synuclein activity and toxicity. They analyzed a library of protein missense variants for relative toxicity in competitive selection in yeast cells. In the headgroup region, mutations of Lys residues, which are expected to interact with acidic residues in anionic lipids, strongly decreased toxicity. Membrane binding of α-synuclein and the formation of a helix in the membrane-bound state of α-synuclein determine the level of α-synuclein toxicity. These results point to the importance of conserved lysine residues in membrane-bound α-synuclein for its toxicity. Such approach allowed the generation of a high-resolution model for the structure and dynamics of the conformational state of α-synuclein that slows yeast growth ^[14].

4. Synuclein's Cellular Functions and Role in Pathology

α-Synuclein is an essential regulator of synaptic vesicle pool and trafficking, dopamine neuro-transmission, and other mechanisms involved in synaptic plasticity ^[15][16]. α-Synuclein performs these functions by assisting in the creation of soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE)-complex ^[15][16][17]. α-Synuclein participates in chaperoning SNARE complex assembly through its interaction with vesicle-associated membrane protein 2 (VAMP2)/synaptobrevin-2 ^[15][16][17].

Contrary to α-synuclein, our knowledge about the role of other synuclein family members, β-synuclein and γ-synuclein, in cellular functions and pathology is somewhat limited. Recent findings about β-synuclein point to its role as an important biomarker for the early stages of Alzheimer’s disease (AD) ^[17]. The concentration of β-synuclein gradually increases in the cerebrospinal fluid beginning from the preclinical AD phase. It may be considered a promising biomarker of synaptic damage in this disease. β-Synuclein may be used as a CSF biomarker for synaptic damage in AD; its level is elevated in both dementia and pre-dementia stages of AD ^[17][18]. Importantly, higher CSF α-synuclein levels are reported in pre-AD subjects but not in MCI-AD and dementia AD [21], pointing to its specificity as a biomarker.