Cell Biology Chapter 13 Flashcards
Cytoskeleton Roles and Structural elements
Structural support; Positioning framework for cellular organelles; Network of tracks for moving organelles and cellular material; Cell movement; Cell division; Structural: Microtubules - are composed of tubulin subunits; Microfilaments (actin) - actin subunits; Intermediate filaments (not in plants) - variable in composition; Septins (in animals and fungi, not in plants) – different septin proteins
Septins Roles and Organization
Cytokinesis, Vesicle trafficking, Nuclear division coordination, Cytoskeleton organization, Polarity maintenance, Compartmentalization of pre-existing cellular material, Formation of diffusion barriers; Organization: Monomers: septin proteins, Form Hetero-oligomeric filaments, Nonpolar filaments, Bind GTP/GDP
Microtubules: Roles, how they are made, polarity, polymerization, drugs that affect it
Roles: Cytoplasmic microtubules spread through the cytosol and are responsible for a variety of functions: Mechanical Work->Move Transport Vesicles; Cell Division->Mitotic Spindle from centrosome; Cell Movement-> Core of Cilia and Flagella; Maintaining axons; Formation of mitotic and meiotic spindles; Maintaining or altering cell shape; Placement and movement of vesicles; How they are made, Their polarity & Polymerization: MTs are straight, hollow cylinders of varied length that consist of (usually 13) longitudinal arrays of polymers called protofilaments; The basic subunit of a protofilament is a heterodimer of tubulin, one α-tubulin and one β-tubulin; These bind noncovalently to form an αβ-heterodimer, which does not normally dissociate; ome drugs causes dividing cells to arrest during mitosis. It can used in cancer treatment; Colchicine binds to tubulin monomers, inhibiting their assembly into MTs and promoting MT disassembly; Nocodazole inhibits MT assembly, and its effects are more easily reversed than those of colchicine; MTs form by the reversible polymerization of tubulin dimers in the presence of GTP and Mg2+; Dimers aggregate into oligomers, which serve as “seeds” from which new MTs grow; This process is called nucleation; the addition of more subunits at either end is called elongation Addition of Tubulin Dimers Occurs More Quickly at the Plus Ends of Microtubules; The two ends of an MT differ chemically, and one can grow or shrink much faster than the other; The rapidly growing MT end is the plus end; The other is the minus end; Treadmilling: addition of subunits at the plus end, and removal from the minus end; The two ends, called the plus end (+) and the minus end (-), differ both chemically and structurally
Roles of GTP/GDP, catastrophe and rescue, MTOC and their organization, and MT stabilizing/ Bundling and Destabilizing and Severing
Each tubulin heterodimer binds two GTP molecules; α-tubulin binds one, and β-tubulin binds a second to organize into protofilament; GTP bound to α-tubulin is not hydrolyzed; GTP bound to β-tubulin is hydrolyzed to GDP after polymerization (β-tubulin is also a GTPase); The GTP bound to the β-subunit is hydrolyzed to GDP after the heterodimer is added to the MT; GTP is needed to promote heterodimer interactions and addition to MTs, but its hydrolysis is not required for MT assembly; Dynamic instability model: one population of MTs grows by polymerization at the plus ends, whereas another population shrinks by depolymerization; Growing MTs have GTP at the plus ends, and shrinking MTs have GDP; The GTP cap at the plus end prevents subunit removal; Rapidly growing MT contain a GTP cap (GTP-bound heterodimers = straight conformation), due to faster rate of addition of α/β heterodimer than the rate of GTP hydrolysis to GDP; This maintains the stability of the growing MT; If GTP-tubulin is high, it is added to an MT quickly, creating a large GTP-tubulin cap; If the concentration falls, the rate of tubulin addition decreases; At a sufficiently low GTP-tubulin, the rate of GTP hydrolysis exceeds the rate of subunit addition, and the cap shrinks; Catastrophe and Rescue: If the GTP cap disappears altogether, the MT becomes unstable, and loss of GDP-bound subunits is favored; Individual MTs can go through periods of growth and shrinkage; a switch from growth to shrinkage is called microtubule catastrophe; A sudden switch back to growth phase is called microtubule rescue ; MTOC and their organization: Microtubules Originate from Microtubule-Organizing Centers: ; MTs originate from a microtubule-organizing center (MTOC); Many cells have an MTOC called a centrosome near the nucleus; In animal cells, the centrosome is associated with two centrioles surrounded by pericentriolar material; Centriole walls are formed by nine pairs of triplet microtubules; Cells without centrioles have poorly organized mitotic spindles; Centrosomes have large ring-shaped protein complexes in them; these contain γ-tubulin; γ-tubulin is found only in centrosomes; γ-tubulin ring complexes (γ-TuRCs) nucleate the assembly of new MTs away from the centrosome; Loss of γ-TuRCs prevents a cell from nucleating MTs; MTOCs Organize and Polarize the Microtubules Within Cells; MTOCs nucleate and anchor MTs; MTs grow outward from the MTOC with a fixed polarity—the minus ends are anchored in the MTOC; of this, dynamic growth and shrinkage of MTs occurs at the plus ends, near the cell periphery; MT stabilizing/Bundling and Destabilizing and Severing MAPs, microtubule-associated proteins, bind at regular intervals along a microtubule wall, allowing for interaction with other cellular structures and filaments; One domain attached to the side of the MT, another domain projects outward; Maintain parallel orientation, increasing stability; The length of the extended “arm” controls the spacing of MTs in the bundle; + TIP Proteins; MTs can be stabilized by proteins that “capture” and protect the growing plus ends; These are +TIP proteins (+ end tubulin interacting proteins); Some proteins promote depolarization of MTs
Microfilaments MFs (actin) Roles, How they are made, polarity, polymerization, drugs that affect it, roles of ATP/ADP, Actin Binding Proteins Regulate the Polymerization, Length, and Organization of Actin
Microfilaments are the smallest of the cytoskeletal filaments; They are best known for their role in muscle contraction; They are involved in cell migration, amoeboid movement, and cytoplasmic streaming; Development and maintenance of cell shape; Actin is a very abundant protein in all eukaryotic cells; Once synthesized, it folds into a globular-shaped molecule that can bind ATP or ADP (G-actin; globular actin); G-actin molecules polymerize to form microfilaments, F-actin; G-actin monomers can polymerize reversibly into filaments with a lag phase and elongation phase, similar to tubulin assembly; F-actin filaments are composed of two linear strands of polymerized G-actin wound into a helix; All the actin monomers in the filament have the same orientation; MFs have a distinctive arrowhead pattern; The plus end of an MF is called the barbed end; The minus end is called the pointed end; Cytochalasins are fungal metabolites that prevent the addition of new monomers to existing MFs; Latrunculin A is a toxin that sequesters actin monomers and prevents their addition to MFs; Phalloidin stabilizes MFs and prevents their depolymerization; Actin-binding proteins are responsible for converting actin filaments from one form to another; (a) Some proteins affect monomer availability or monomer addition; (b) Others affect severing or growth of existing filaments; (c) still others affect filament organization; Whether MFs can grow depends on whether their filament ends are capped; Capping proteins bind the ends of a filament to prevent further loss or addition of subunits; Often, actin networks form as loose networks of crosslinked filaments; One of the proteins important in the formation of these networks is filamin; Filamin acts in joining two MFs together where they intersect; MFs are broken up by proteins that sever and/or cap them; Gelsolin breaks actin MFs and caps the newly exposed plus ends, preventing further polymerization; Profilin is an abundant regulator of actin dynamics that supports filament assembly at barbed ends by binding G-actin; Actin can also form a tree-like network; A complex of actin-related proteins, the Arp2/3 complex, nucleates new branches on the sides of filaments; In this case, actin polymerization is regulated independently of the Arp2/3 complex, through proteins called formins; Formins move along the end of the growing filament as they promote polymerization
Intermediate Filaments
Intermediate filaments (IFs) are not found in cytosol of plant cells but are abundant in many animal cells; IFs are the most stable and least soluble components of the cytoskeleton; They likely support the entire cytoskeleton; Intermediate Filaments Confer Mechanical Strength on Tissues; IFs are less susceptible to chemical attack than are MTs and MFs; Provide strength (e.g. hair, sarcomere, nuclear matrix); Involved in cell junctions; NOT involved in intracellular transport; IFs differ greatly in amino acid composition from tissue to tissue; They are grouped into six classes
Intermediate Filaments Assemble from Fibrous Subunits; The fundamental subunits of IF proteins are dimers; IF proteins are fibrous rather than globular; Each has a homologous central rod-like domain; Flanking the central helical domain are N- and C-terminal domains that differ greatly among IF proteins; Intermediate filament: Polarity, Nucleation and Disruption; Do NOT have polarity; No known motor proteins; Not directly involved in cellular movement; Nucleation believed to occur upon formation of staggered tetramer; Polymerization and disruption controlled by phosphorylation, does NOT require GTP or ATP; monomers link into dimer; dimers link into staggered tetramer; tetramers link into strand; 8 tetramer strands link into filament; Subunits can be added or removed from the middle of a filament, and are not dependent on ends for growth. Examples: Keratin: Found in desmosomes and hair; Desmin: Found in muscle sarcomere; Lamin: Found in nuclear matrix
How are cytoskeletons connected?
Microtubules resist bending when a cell is compressed; Microfilaments serve as contractile elements that generate tension; Intermediate filaments are elastic and can withstand tensile forces; Spectraplakins are linker proteins that connect intermediate filaments, microfilaments, and microtubules
