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Lecture 19

BIOL 3113 Lecture Notes - Lecture 19: Globular Protein, Desmosome, Tubulin

Course Code
BIOL 3113
Barbara S

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19 Cytoskeleton
The cytoskeleton is an intracellular network of filament-forming proteins that control:
*Cell shape
*Internal organization and intracellular movements
Three families of cytoskeletal filaments:
*Microfilaments (7-9 nm diameter)
composed of actin
*Microtubules (20-25 nm diameter)
composed of tubulin
*Intermediate filaments (10 nm diameter)
composed of various genetically related proteins
Cytoskeletal accessory proteins
Various other proteins associated with the cytoskeleton that:
*connect cytoskeletal filaments to each other
*connect cytoskeletal filaments to other cellular components
*regulate assembly and disassembly
Intermediate Filaments - found in most animal cells
Tough, insoluble rope-like protein filaments, form a network extending from the
perinuclear region to the cell periphery, often anchor to cell membrane at desmosomes
(cell junctions)
*Very stable:
Exception: disassembly of nuclear lamin filaments during cell division
*Virtually insoluble
*High mechanical strength
Structure of Intermediate Filaments
*Monomers - elongated fibrous proteins:
globular amino-terminal head
central rod
globular carboxyl-terminal tail
*Dimers - two subunits associate side-by-side and coil around each other to form a coiled
*Tetramers - two dimers associate in a staggered antiparallel manner => native subunit

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*Filament - large numbers of tetramers associated end-to-end and side-by-side in a
helical manner to form a continuous filament
Four families of Intermediate Filaments:
very diverse group of proteins (found in epithelial cells)
*Vimentin and vimentin-related proteins
vimentin - cells of mesodermal origin
desmin - muscle cells
glial fibrillary acidic protein - glia cells
*Neurofilament proteins - neurons
*Nuclear lamins - form the nuclear lamina
Functions of Intermediate Filaments
*Provide mechanical stability to cells
Increase tensile strength of the cell (help cells resist shearing under mechanical stress)
Increase mechanical toughness
Mutations in keratin genes - epidermal layer not strongly attached to the basement
membrane (e.g., epidermolysis bullosa simplex - severe skin blistering)
Surround and support the nuclear envelope
*Help to organize the internal structure of cells
Determine the spatial organization of the cytosol
Determine the location of the cell nucleus
*Involved in the control of cell polarity
Confer organization inside the cell and provide tracks for intracellular transport; form
mitotic spindle, cilia and flagella
Long, relatively stiff, hollow cylinders composed of the protein tubulin
*Monomers - globular proteins of two types
alpha tubulin
beta tubulin
composed of one alpha and one beta monomer
native subunit for tubule assembly

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dimers assemble end-to-end and side-by-side in a spiral with a hollow center
the microtubule wall contains 13 protofilaments with alternating alpha and beta tubulins
Microtubule - Growth and Polarity
Microtubules can grow by adding subunits to either end and shrink by losing subunits
from either end
*Plus end
assembles faster
disassembles slower
*Minus end
assembles slower
disassembles faster
often capped to prevent disassembly and stabilize MT's
MT grow from an initial ring of 13 dimers - nucleating centers for MT assembly
The rate-limiting step of MT assembly - formation of nucleating centers
Cells have preassembled nucleating centers called microtubule organizing centers
The MTOC also caps the minus end of the MT, limiting growth to the plus end and
preventing disassembly
MTOC in cells:
*centrosome (cytosolic and spindle MT)
contains rings of gamma tubulin - nucleation sites
contains a pair of centrioles (animals)
*basal bodies (cilia and flagella)
*kinetochores (chromosome MT)
Dynamic Instability of Microtubules
Microtubule growth is not smooth. MT may grow steadily for several minutes, then
rapidly shrink, then grow again - dynamic stability
Caused by the binding and hydrolysis of GTP by tubulin
Tubulin dimers bind one molecule of GTP
Tubulin-GTP dimers bind more tightly than tubulin-GDP
Once dimers polymerize, the GTP will hydrolyze at random to GDP
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