Cytoplasm, Cytosol, Ubiquitin-Proteasome System, and Inclusion Bodies

Comprehensive vocabulary flashcards covering cytoplasm organization, cytosol composition, the ubiquitin-proteasome system (UPS), mitophagy, and cellular inclusion bodies (storage substances, endogenous pigments, and crystals) from Cell Biology Topic 9.

Cytoplasm

The cellular compartment comprising cytosol and organelles that can interconvert between two physical states: a fluid sol state and a more rigid gel state.

Sol-to-Gel and Gel-to-Sol Transitions

Localized, reversible transitions in the physical state of the cytoplasm that generate the forces and cytoplasmic streaming necessary for cell movement and migration.

Cytosol

The semi-fluid intracellular substance composed of approximately $80\%$ water, ions, carbohydrates, lipids, amino acids, proteins, nucleotides, and RNAs, which serves as the site for metabolic pathways, signaling, and cell division.

Ubiquitin-Proteasome System (UPS)

A major non-lysosomal protein degradation complex measuring approximately $50 \times 10\,\text{nm}$ that hydrolyzes peptide bonds in damaged, misfolded, or short-lived proteins tagged covalently with ubiquitin.

20S Proteasome

The core proteolytic cylinder of the proteasome with a molecular mass of $700\,\text{kDa}$ that possesses proteolytic active sites lined with three N-terminal threonine proteases on its inner chamber.

19S Regulator

A multiprotein regulatory complex composed of a base and a lid that binds to the $20\text{S}$ core in an ATP-dependent manner to recognize polyubiquitinated substrates, unfold them, and open the gate to the proteolytic chamber.

26S Proteasome

A functional $2000\,\text{kDa}$ protein degradation machine assembled from the $20\text{S}$ catalytic core capped by one or two $19\text{S}$ regulatory complexes in the presence of ATP.

Ubiquitin (Ub)

A small regulatory protein attached via an enzymatic cascade involving E1, E2, and E3 enzymes to target specific proteins for degradation by the $26\text{S}$ proteasome.

PINK1

A mitochondrial serine/threonine-protein kinase that accumulates and auto-phosphorylates on depolarized mitochondria, phosphorylating both Parkin and ubiquitin at Ser65 to trigger mitophagy.

Parkin

An E3 ubiquitin ligase that is activated by PINK1-dependent phosphorylation, forms a Parkin-ubiquitin thioester intermediate at Cys431, and ubiquitinates mitochondrial surface substrates.

p62

An adaptor protein that acts to quarantine damaged, depolarized mitochondria and facilitate their degradation through autophagy and proteasomal pathways.

Mitophagy

The selective degradation of damaged or depolarized mitochondria by the proteasome and the autophagy-lysosome system following PINK1- and Parkin-mediated ubiquitylation.

Autophagoproteasome

A hybrid intracellular structure formed through the direct interplay and convergence of the ubiquitin-proteasome system and autophagosomes.

IκB

An inhibitory protein whose targeted degradation by the ubiquitin-proteasome system releases the transcription factor $NF\text{-}\kappa B$, allowing it to enter the nucleus and induce inflammatory gene expression.

Human Papillomavirus (HPV) E3 Ligase

A viral enzyme that directs the ubiquitin-mediated degradation of the tumor suppressor protein p53, facilitating cell survival and cervical tumor development.

Inclusion Bodies

Non-membrane-bound cytoplasmic structures comprising stored nutrients, metabolic byproducts, pigments, or crystalline precipitates.

Glycogen Granules

Cytoplasmic carbohydrate storage inclusions abundant in hepatocytes and muscle cells, organized hierarchically from $\gamma$-particles ($1\,\text{nm}$) into $\beta$-granules ($10\,\text{nm}$) and rosette-like $\alpha$-granules ($50\,\text{nm}$).

Lipid Droplets

Spherical intracellular storage inclusions of neutral lipids, commonly found in white adipose tissue, brown adipose tissue, and steatotic liver cells.

Lipofuscin

A golden-brown 'wear-and-tear' pigment within residual bodies consisting of oxidized protein and lipid polymers produced by free radical-mediated peroxidation during aging, prominent in aged cardiomyocytes, hepatocytes, and neurons.

Melanin

An endogenous brown-to-black or yellow-to-red pigment synthesized within melanosomes of melanocytes and transferred to keratinocytes to provide photoprotection.

Tyrosinase

A key melanocyte enzyme that catalyzes the initial steps of melanin synthesis, converting tyrosine to dopa and subsequently to dopaquinone.

Ephelides

Small, flat, pigmented skin macules commonly known as freckles, resulting from localized increases in melanin production by normal numbers of melanocytes.

Vitiligo

A skin disorder characterized by sharply circumscribed depigmented patches resulting from the loss or destruction of functional epidermal melanocytes.

Lentigo

A benign pigmented spot on the skin caused by a localized proliferation and hyperplasia of melanocytes along the basal cell layer.

Melanocytic Nevus

A benign circumscribed pigmented skin lesion composed of clusters of proliferating melanocytes, commonly known as a mole.

Melanoma

A malignant neoplasm arising from the transformation of melanocytes, characterized by asymmetrical, irregular, and deeply pigmented lesions.

Retinal Pigment Epithelium (RPE)

A pigmented monolayer of epithelial cells located in the outer retina that contains melanin, lipofuscin, and melanolipofuscin granules, providing metabolic support and photoprotection to photoreceptors.

Hemosiderin

A golden-yellow to brown iron-storage complex derived from heme breakdown, frequently accumulating in macrophages (such as alveolar macrophages in chronic passive congestion), hepatocytes, and renal tubular cells.

Bile Pigments

Endogenous green-to-brown pigments produced by hepatocytes during bilirubin metabolism that accumulate pathologically in liver tissue during cholestasis.

Uric Acid Crystals

Crystalline inclusion bodies found within synovial leukocytes in joints affected by gout, identifiable under polarized light as needle-like birefringent deposits.