Mod4 - The Cytoskeleton and Molecular Motors Flashcards
(47 cards)
In what groups of organisms are the following found - actin microfilaments, microtubules, intermediate filaments?
Actin microfilaments = all eukaryotes; MTs = all eukaryotes, Intermediate Filaments = most animals
Describe the structure of microtubules
Unbranched cylinders assembled from α/ß tubulin heterodimers (25nm diameter)
Describe the “polar” nature of microtubules
“Plus ends” grow rapidly, with ß-tubulin exposed; “minus ends” grow slowly, if at all
How well can spontaneous MT formation occur in a test tube/in a cell?
TT: Tubulin + GTP + Mg2+ -> (37C) Microtubules!
Cell: Tubulin concentration too low for polymerisation to occur spontaneously
How do cells speed up MT polymerisation since tubulin concentration is low?
They use a template made of gamma-tubulin and other proteins (this is called nucleation)
How does nucleation speed up MT formation?
Spontaneous growth involves more unfavourable (slow) steps than growth from a preformed nucleus
Gamma tubulin rings are concentrated on specific structures - name two examples
Centromeres and Basal Bodies
Where in the cell are the plus and minus ends of MTs?
Plus ends at the periphery; minus ends at the cell centre (centrosome)
Describe the nature/order of growing and shrinking seen in MTs
Each MT grows and shrinks independently of its neighbours, and can switch between growing and shrinking (dynamic instability)
Describe the role of GTP in microtubules
Tubulin is a GTPase: GDP tubulin cannot polymerise, but GTP tubulin can. In a microtubule, GTP is gradually hydrolysed back to GDP
Describe the effect of the presence/absence of the GTP cap on MT growth
If a GTP cap is present, the MT will continue to grow; if the GTP cap is lost, MT will depolymerise; if a new GTP cap forms, the MT will start growing again
What can stabilise microtubules (3 things)?
Holding Microtubule-Associated Proteins (MAPs) all along the molecule; binding the drug Taxol; capture of MT plus ends by proteins at the cell cortex
How can MTs be experimentally depolymerised? (2 ways)
Putting cells on ice; using drugs that bind free tubulin dimers, preventing new assembly (e.g., Nocodazole, Colcemid, Colchicine)
Describe the structure of actin filaments
Thin, flexible, helical filaments, assembled from monomeric actin (7nm diameter)
Describe the polar nature of actin filaments
They have a plus end (ATP-bound actin) and a minus end (ADP-bound actin)
Name three natural small molecules which can alter actin polymerisation (and how)
- Phalloidin - stabilises AFs
- Cytochalasin - caps filament ends, preventing actin polymerisation from existing ends
- Latrunculin - binds to actin monomers, preventing actin polymerisation
What effect does formin have on actin filaments?
It is a Nucleating Protein - promotes polymerisation
What effect does the Arp2/3 complex have on actin filaments?
It alters filament length/dynamics
Name 4 types of proteins that alter actin filament organisation
Severing, cross-linking, bundling and capping proteins
Name two types of proteins that control or drive movement along actin filaments
Site-binding protein, motor protein
Name the two neuronal MAPs (microtubule-associated proteins) and one non-neuronal one
Neuronal: MAP2 and Tau
Non-Neuronal: MAP4
Name the type of motor protein associated with actin filaments
Myosins
What are the two actin-based structures which probe the environment at the leading edge of migrating cells?
Lamellipodia and Filopodia
What does Arp2/3 complex do in Lamellipodia?
Binds to the side of existing actin filaments, causing branching - thus nucleating the assembly of new actin filaments AND preventing disassembly from the minus end
