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Structural proteins

Uni of Notts, Structure, function, & analysis of Proteins, year 2, topic 10

TermDefinition
Structural proteins Filamentous, often insoluble proteins specialised for intracellular or extracellular mechanical support. Often have unusual patterns of amino acids & a lot of cross linkage
Three major cytoskeletal components Actin microfilaments (7nm), microtubules (25nm), & intermediate filaments (10nm)
Persistence length (Lp) Measure of filament rigidity; larger persistence length means greater stiffness & resistance to bending
Lp compared to length (L) for each cytoskeleton component Microtubules: Lp > L, very rigid Actin: Lp ~ L, semi-flexible Intermediate filaments: LP < L, very flexible
Actin filament formation G-actin (globular) monomers polymerise into F-actin (filamentous) filaments through nucleation & elongation
Actin nucleation Formation of stable actin dimers/trimers that initiate polymerisation
Actin treadmilling ATP-actin adds mainly to the barbed + end while ADP-actin dissociates from the pointed − end
Role of the ARP2/3 complex ARP2/3 nucleates branched actin filaments, especially in lamellipodia (projections migrating neurones or immune cells use) & motile structures. Binds favourably near + end
Cofilin & profilin (& thymosin-β4) Cofilin promotes ADP-actin disassembly; profilin exchanges actin ADP for ATP to promote assembly. Thymosin-β4 sequesters profilin for rapid deployment if necessary
Key functions of actin filaments Cell movement, cytokinesis, shape maintenance, adhesion, & microvilli support
Microtubules composition α- & β-tubulin heterodimers assembled into hollow filaments nucleating from centrosomes near the nucleus
GTP cap in microtubules Region of microtubule where GTP remains unhydrolysed which stabilises growing microtubules & prevents rapid depolymerisation (catastrophy)
Why microtubules use GTP instead of ATP Using different nucleotides separates regulation of actin & microtubule dynamics; & keeps microtubule structure independant of general cell energy needs
Main functions of microtubules Cell shape, intracellular transport, mitosis, organelle positioning, & motility
Intermediate filaments assembly Coiled dimers dimerise to form tetramers, 8 tetramers form a sheet, sheets anneal to rope-like filament bundles
Structural strength in intermediate filaments Coiled α-helical structure allows flexibility & resistance to tensile stress
Cell-line specificity of intermediate filaments Different IF proteins are expressed depending on cellular mechanical & functional needs in different specialised cell lines
Keratin & subtypes Major intermediate filament structural protein found in skin, feathers, fur, & related structures. Either formed from α or β 2* structures or both leading to α, β, or α&β subtypes
Keratin most-common residues Cys for many disulphide bridges to strengthen & stiffen. Glu & Arg for salt bridges. Gln for cross-linkage. Ser for hydration
Heptad repeat in α-keratin Repeating 7-residue pattern stabilising coiled-coil dimers through hydrophobic & ionic interactions. Hydrophobic residues pack internally, charged residues form salt bridges between helices
β-keratin vs α-keratin β-keratin is β-sheet rich & forms rigid structures like feathers & scales
Super-secondary structure Recurring arrangement of secondary structures forming functional motifs/domains
Created by: Denny12
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