PSMB8

Proteasome subunit beta type-8 as known as 20S proteasome subunit beta-5i is a is_associated_with::protein that in humans is encoded by the PSMB8 is_associated_with::gene. This protein is one of the 17 essential subunits (alpha subunits 1-7, constitutive beta subunits 1-7, and inducible subunits including beta1i, beta2i, beta5i) that contributes to the complete assembly of 20S is_associated_with::proteasome complex. In particular, proteasome subunit beta type-5, along with other beta subunits, assemble into two heptameric rings and subsequently a proteolytic chamber for substrate degradation. This protein contains "Chymotrypsin-like" activity and is capable of cleaving after large hydrophobic residues of peptide. The eukaryotic is_associated_with::proteasome recognized degradable proteins, including damaged proteins for protein quality control purpose or key regulatory protein components for dynamic biological processes. The constitutive subunit beta1, beta2, and beta 5 (systematic nomenclature) can be replaced by their inducible counterparts beta1i, 2i, and 5i when cells are under the treatment of interferon-γ. The resulting proteasome complex becomes the so-called immunoproteasome. An essential function of the modified proteasome complex, the immunoproteasome, is the processing of of numerous MHC class-I restricted T cell epitopes.

Gene
This gene encodes a member of the proteasome B-type family, also known as the T1B family, that is a 20S core beta subunit. This gene is located in the class II region of the MHC (major histocompatibility complex). Expression of this gene is induced by is_associated_with::gamma interferon and this gene product replaces catalytic subunit 3 (proteasome beta 5 subunit) in the immunoproteasome. Proteolytic processing is required to generate a mature subunit. Two alternative transcripts encoding two isoforms have been identified; both isoforms are processed to yield the same mature subunit. The human PSMB8 gene has 7 exons and locates at chromosome band 6p21.3.

Protein structure
The human protein proteasome subunit beta type-8 is 23 kDa in size and composed of 204 amino acids. The calculated theoretical pI of this protein is 7.59.

Complex assembly
The is_associated_with::proteasome is a multicatalytic proteinase complex with a highly ordered 20S core structure. This barrel-shaped core structure is composed of 4 axially stacked rings of 28 non-identical subunits: the two end rings are each formed by 7 alpha subunits, and the two central rings are each formed by 7 beta subunits. Three beta subunits (beta1, beta2, beta5) each contains a proteolytic active site and has distinct substrate preferences. Proteasomes are distributed throughout eukaryotic cells at a high concentration and cleave peptides in an ATP/is_associated_with::ubiquitin-dependent process in a non-is_associated_with::lysosomal pathway.

Function
Protein functions are supported by its tertiary structure and its interaction with associating partners. As one of 28 subunits of 20S proteasome, protein proteasome subunit beta type-2 contributes to form a proteolytic environment for substrate degradation. Evidences of the crystal structures of isolated 20S proteasome complex demonstrate that the two rings of beta subunits form a proteolytic chamber and maintain all their active sites of proteolysis within the chamber. Concomitantly, the rings of alpha subunits form the entrance for substrates entering the proteolytic chamber. In an inactivated 20S proteasome complex, the gate into the internal proteolytic chamber are guarded by the is_associated_with::N-terminal tails of specific alpha-subunit. This unique structure design prevents random encounter between proteolytic active sites and protein substrate, which makes protein degradation a well-regulated process. 20S proteasome complex, by itself, is usually functionally inactive. The proteolytic capacity of 20S core particle (CP) can be activated when CP associates with one or two regulatory particles (RP) on one or both side of alpha rings. These regulatory particles include 19S proteasome complexes, 11S proteasome complex, etc. Following the CP-RP association, the confirmation of certain alpha subunits will change and consequently cause the opening of substrate entrance gate. Besides RPs, the 20S proteasomes can also be effectively activated by other mild chemical treatments, such as exposure to low levels of sodium dodecylsulfate (SDS) or NP-14.

The 20S proteasome subunit beta-5i (systematic nomenclature) is originally expressed as a precursor with 276 amino acids. The fragment of 72 amino acids at peptide N-terminal is essential for proper protein folding and subsequent complex assembly. At the end-stage of complex assembly, the N-terminal fragment of beta5i subunit is cleaved, forming the mature beta5i subunit of 20S complex. During the basal assembly, and proteolytic processing is required to generate a mature subunit. The subunit beta5i only presents in the immunoproteasome and is replaced by subunit beta5(proteasome beta 5 subunit) in constitutive 20S proteasome complex.

Clinical significance
The Proteasome and its subunits are of clinical significance for at least two reasons: (1) a compromised complex assembly or a dysfunctional proteasome can be associated with the underlying pathophysiology of specific diseases, and (2) they can be exploited as drug targets for therapeutic interventions. More recently, more effort has been made to consider the proteasome for the development of novel diagnostic markers and strategies. An improved and comprehensive understanding of the pathophysiology of the proteasome should lead to clinical applications in the future.

The proteasomes form a pivotal component for the Ubiquitin-Proteasome System (UPS) and corresponding cellular Protein Quality Control (PQC). Protein is_associated_with::ubiquitination and subsequent is_associated_with::proteolysis and degradation by the proteasome are important mechanisms in the regulation of the is_associated_with::cell cycle, is_associated_with::cell growth and differentiation, gene transcription, signal transduction and is_associated_with::apoptosis. Subsequently, a compromised proteasome complex assembly and function lead to reduced proteolytic activities and the accumulation of damaged or misfolded protein species. Such protein accumulation may contribute to the pathogenesis and phenotypic characteristics in neurodegenerative diseases, cardiovascular diseases,   inflammatory responses and autoimmune diseases,  and systemic DNA damage responses leading to is_associated_with::malignancies.

Several experimental and clinical studies have indicated that aberrations and deregulations of the UPS contribute to the pathogenesis of several neurodegenerative and myodegenerative disorders, including is_associated_with::Alzheimer's disease, is_associated_with::Parkinson's disease and is_associated_with::Pick's disease, is_associated_with::Amyotrophic lateral sclerosis (is_associated_with::ALS), is_associated_with::Huntington's disease, is_associated_with::Creutzfeldt-Jacob disease, and motor neuron diseases, polyglutamine (PolyQ) diseases, is_associated_with::Muscular dystrophies and several rare forms of neurodegenerative diseases associated with is_associated_with::dementia. As part of the Ubiquitin-Proteasome System (UPS), the proteasome maintains cardiac protein homeostasis and thus plays a significant role in cardiac is_associated_with::Ischemic injury, is_associated_with::ventricular hypertrophy and is_associated_with::Heart failure. Additionally, evidence is accumulating that the UPS plays an essential role in malignant transformation. UPS proteolysis plays a major role in responses of cancer cells to stimulatory signals that are critical for the development of cancer. Accordingly, gene expression by degradation of is_associated_with::transcription factors, such as is_associated_with::p53, is_associated_with::c-Jun, is_associated_with::c-Fos, is_associated_with::NF-κB, is_associated_with::c-Myc, HIF-1α, MATα2, is_associated_with::STAT3, sterol-regulated element-binding proteins and is_associated_with::androgen receptors are all controlled by the UPS and thus involved in the development of various malignancies. Moreover, the UPS regulates the degradation of tumor suppressor gene products such as is_associated_with::adenomatous polyposis coli (APC) in colorectal cancer, is_associated_with::retinoblastoma (Rb). and is_associated_with::von Hippel-Lindau tumor suppressor (VHL), as well as a number of is_associated_with::proto-oncogenes (is_associated_with::Raf, is_associated_with::Myc, Myb, Rel, Src, Mos, Abl).The UPS is also involved in the regulation of inflammatory responses. This activity is usually attributed to the role of proteasomes in the activation of NF-κB which further regulates the expression of pro inflammatory is_associated_with::cytokines such as is_associated_with::TNF-α, IL-β, IL-8, is_associated_with::adhesion molecules (is_associated_with::ICAM-1, is_associated_with::VCAM-1, P selectine) and is_associated_with::prostaglandins and is_associated_with::nitric oxide (NO). Additionally, the UPS also plays a role in inflammatory responses as regulators of leukocyte proliferation, mainly through proteolysis of cyclines and the degradation of CDK inhibitors. Lastly, is_associated_with::autoimmune disease patients with SLE, is_associated_with::Sjogren's syndrome and is_associated_with::rheumatoid arthritis (RA) predominantly exhibit circulating proteasomes which can be applied as clinical biomarkers.

During the antigen processing for the major histocompatibility complex (MHC) class-I, the proteasome is the major degradation machinery that degrades the antigen and present the resulting peptides to cytotoxic T lymphocytes. The immunoproteasome has been considered playing a critical role in improving the quality and quantity of generated class-I ligands.

The PSMB8 protein has a significant clinical role in is_associated_with::autoimmune diseases and is_associated_with::inflammatory reactions. For instance, patients with a homozygous is_associated_with::missense mutation (G197V) in the is_associated_with::immunoproteasome subunit, β type 8 (PSMB8) suffered from autoinflammatory responses that included recurrent fever and nodular is_associated_with::erythema together with is_associated_with::lipodystrophy. This mutation increased assembly intermediates of immunoproteasomes, resulting in decreased proteasome function and ubiquitin-coupled protein accumulation in the patient's tissues. In the patient's skin and is_associated_with::B cells, IL-6 was also highly expressed, and there was a reduced expression of PSMB8. Furthermore, downregulation of PSMB8 also inhibited the differentiation of murine and human is_associated_with::adipocytes in vitro, while an injection of is_associated_with::siRNA against Psmb8 in mouse skin could reduce adipocyte tissue volume. Thus, PSMB8 may be an essential component and regulator not only for inflammation, but also in the differentiation of adipocytes, hereby indicating that immunoproteasomes may have pleiotropic functions to maintain the homeostasis of a variety of cell types. Subsequently, in addition to autoimmune diseases the PSMB8 protein also has been linked in the diagnosis of lipodystrophy syndrome. Glycosylation disorders are sometimes involved. Some genetically determined forms have recently been found to be due to autoinflammatory syndromes linked to a proteasome anomaly through PSMB8. They result in a lipodystrophy syndrome that occurs secondarily with fever, is_associated_with::dermatosis and is_associated_with::panniculitis, and is_associated_with::Nakajo-Nishimura syndrome, a distinct inherited inflammatory and wasting disease that is originated from Japan. Patients with Nakajo-Nishimura syndrome, develop periodic high fever and nodular erythema-like eruptions, and gradually progress lipomuscular is_associated_with::atrophy in the upper body, mainly the face and the upper extremities, to show the characteristic thin facial appearance and long is_associated_with::clubbed fingers with is_associated_with::joint contractures.