Nem’s Notes…
Phase 2 Year 3
MOLECULES, CELLS & TISSUES 8 (page 1 of 1)
Control of Cell Motility
Ca2+ Control
Mechanisms
Control of cell motility is imporrtant to organise cellular activity to respons to external
influences. In skeletal muscle the nervous system uses Ca 2+ as the main second
messenger binding to troponin (which is described above). Smooth muscle is also
controlled by Ca2+ via Ca2+-dependent phosphorylation of the myosin light chain.
Actin filaments can also be regulated by Ca2+. This occurs via accessory proteins
such as gelsolin which is Ca2+-dependent and severs actin filaments, breaking rigid
crosslinked structures. α-actinin acts to crosslink actin filaments and is inhibited by
high [Ca2+].
PIP2 Control
Mechanisms
Some proteins are bound to PIP2 and retained at the plasma membrane until they are
released by hydrolysis during signalling. These include:
(a) Gelsonin (Ca2+-dependent severing of actin)
(b) Profilin (Binds G-actin)
(c) Cofilin (Promotes depolymerisation)
G Protein
Control
Some small G proteins related to Ras can exert control over the actin cytoskeleton.
These proteins include :
(a) Rac (causes membrane ruffling)
(b) Rho (forms actin bundles)
(c) Cdc42 (forms membrane spikes)
These are probably required for cell locomotion.
Cell
Locomotion
Cells can sense a chemotactic gradient of less than 1% from the front to the back of
the cell. The sequence of steps involved and the probable causes are shown below:
(a) Extension of front end (?Rac, actin depolymerisation, repolymerisation)
(b) Adhesion to surface (?Rho)
(c) Translocation of cell contents forward (?motor proteins)
(d) De-adhesion at rear (?Ca2+-dependent)
(e) Retraction of back of cell (?myosin-II)
Microtubule
Control
Microtubules are dynamically unstable but can be stabilised by microtubuleassociated proteins (MAPs) which occurs in nerve axons. Phosphorylation of these
proteins prevents their action and promotes disassembly. Microtubules in the mitotic
spindle are controlled accurately to ensure perfect separation, although the
mechanism is unknown.
Intermediate
Filaments
Intermediate filaments are generally more structural than dynamic. It has associated
proteins which organise the filaments into networks, bundles and attachments to other
cell components. Phosphorylated intermediate filament subunits disassemble during
mitosis. Microtubule disassembly also results in IF collapse.
more online at http://homepage.virgin.net/nemonique.sam/noteindx.htm page 1 of 1