[suivi sur fr.bio.general]


Première fois que je poste sur un groupe bio.xxx, salutations.


Il y a quelques jours, un contributeur de fr.rec.arts.litterature (un endroit où
il est, de fait, rarement question de littérature !) a envoyé ce lien :

http://www.koreus.com/media/inner-life-cell.html


Quelqu'un lui a répondu en substance que c'était effectivement très beau, mais
d'un intérêt modéré si l'on ne savait pas précisément ce que l'on voyait.
http://groups.google.fr/group/fr.rec.arts.litterature/browse_thread/thread/aa61c
b29b9a2f5e7/416885a73502021c


J'ai alors trouvé sur un réseau bien connu de partage de fichiers la version
longue et commentée (en langue de chat qu'espère, oeuf corse) de cette vidéo, et
me suis amusé à retranscrire intégralement le commentaire. N'ayant pas eu
d'écho, même de la part de l'initiateur du fil (il est vrai quelque peu
lunatique), je me permets de recopier ça ici ; tous compléments, corrections,
commentaires bienvenus. Je pourrais également me lancer dans une traduction,
avec l'aide éventuelle des autres intervenants. (Et pourquoi pas créer un
fichier sous-titré qui serait mis en partage de la même manière.)

Voici :


ed2k://|file|Siggraph.2006.Inner.life.of.a.cell.-.long.version.mp4|77704370|9AFA
09BC4B1EDAAAB95DC857EE250C14|/

[Texte retranscrit à l'oreille. Suivi d'un [?], les mots dont je ne suis pas
sûr, ou que j'ai dû rechercher à tâtons, un [???] remplaçant les mots non
identifiés.]

« While red blood cells are carried away at high velocity by a strong blood
flow, leukocytes roll slowly on endothelial cells. P-selectin and endothelial
cells interact with pSGL1 -- a glycoprotein on leukocyte microvilli [?].
Leukocytes pushed by the blood flow adhere and roll on endothelial cells because
existing interactions are broken, while new ones are formed. These interactions
are possible because the extended extracellular domains of both proteins emerge
from the extracellular matrix, which covers the surface of both cell types. The
outer leaflet [?] of the lipid bind layer is rich in sphingolipids and
phosphatidylcholine. Sphingolipids-rich rafts, raised above the rest of the
leaflet [?], recruit specific membrane proteins. Rafts' rigidity is caused by
the tight packing of cholesterol molecules against the straight sphingolipid
hydrocarbon [?] chains. Outside the rafts, [???] unsaturated hydrocarbon chains
and lower cholesterol concentrations result in increased fluidity. At sites of
inflammation, secreted chemokines, bound to heparan-sulfate [?] proteoglycan on
endothelial cells, are presented to leukocyte seven-transmembrane [?] receptors.
The binding stimulates leukocytes and triggers an intracellular cascade of
signaling reactions. The inner leaflet of the biolayer [?] has a very different
composition than that of the outer layer. While some proteins traverse the
membrane, others are rather ancred into the inner leaflet by covalantly attached
fatty acid chains, or are recruited through non-covalent interactions with
membrane proteins. The membrane-bound protein comlexes are critical for the
transmission of signals across the plasma membrane. Beneath [?] the lipid
biolayer, spectrin tetramers, arranged into a hexagonal network, are ancred by
membrane proteins. This network forms the membrane skeleton that contributes to
membrane stability, and membrane protein distribution. The cytoskeleton is
comprised of networks of filamentous proteins that are responsible for the
special organisation of cytosolic components. Inside microvilli, actin filaments
form tight parallels bundles [?], which are stabilized by cross-linking
proteins -- while deeper inside the cells, the actin networks adopt a gel-like
structure, stabilized by a variety of actin-binding proteins. Filaments, [???
peut-être "capturate"] their [??? peut-être "minescence"] by a protein complex,
grow away from the plasma membrane by the addition of actin monomers to their
"+" end. The actin network is a very dynamic structure, with continuous
directional polymerization, and dissassembly. Severing proteins induce kinks [?]
in the filement, and lead to the formatin of short fragments that rapidly
depolymerize -- or give rise to new filaments. The cytoskeleton includes a
network of microtubules created by the lateral association of proto-filaments,
formed by the polymerization of tubulin dimers. While the "+" end of some
microtubules extend toward the plasma membrane, proteins stabilize the curved
conformation of proto-filaments from other microtubules, causing their rapid "+"
end depolymerization. Microtubules provide tracks along which membrane-bound
vesicles travel to-and-from the plasma membrane. The direction of mouvement of
these cargo vesicles is due to a family of motor proteins linking vesicles and
microtubules. Membrane-bound organelles -- like mitochondria -- are loosely
trapped by the cytoskeleton. Mitochondria change shape continuously, and their
orientation is partly dictated by their interaction with microtubules. All the
microtubules originate from the centrosome -- a discrete fiber [?] structure
containing two [??? semble décrire la forme : "...gonal"] centrioles, and
located near the cell nucleus. Pores in the nuclear enveloppe allow the import
of particles containing mRNA and proteins into the cytosol. Here, free ribosoms
translate the mRNA molecules into proteins. Some of these proteins will reside
in the cytosol ; other will associate with specialized cytosolic proteins and be
imported into mitochondria or other organelles. The synthesis of cell-secreted
and integral membrane proteins is initiated by free ribosoms, which then [???
peut-être "duck"] to protein translocator at the surface of the endoplasmic
ruticulum. Nascent proteins pass through an aquious pore in the translocater.
Cell-secreted proteins accumulate in the lumen of the endoplasmic reticulum,
while integral membrane proteins become embedded in the endoplasmic reticulum
membrane. Proteins are transported from the endoplasmic reticulum to the Golgu
apparatus by vesicles travelling along the microtubules. Protein glycosylation,
initiated in the endoplasmic reticulum, is completed inside the lumen of the
Golgi apparatus. Fully glycosylated proteins are transported from the Golgi
apparatus to the plasma membrane. When the vesicles fuses with the plasma
membrane, proteins contained in the vesicle's lumen are secreted, and proteins
embedded in the vesicle's membrane difuse in the cell membrane. At sites of
inflammation, chemokine secreted by endothelial cells bind to the extracellular
domains of G-protein coupled membrane receptors. This binding causes a
conformational change in the cytosolic portion of the receptor, and the
consequent activation of a sub-unit of the G-protein. The activation of the
G-protein sub-unit triggers a cascade of protein activation, which in turn leads
to the activation and clustering [?] of integrins inside lipid rafts. A dramatic
conformational change occurs in the extracellular domain of the activated
integrins. This now allows for the interaction with I-Cam proteins displayed at
the surface of the endothelial cells. These strong interactions immobilize the
rolling leukocyte at the site of inflammation. Additionnal signaling events
cause the profund reorganisation of the cytoskeleton, resulting in the spreading
of one edge [?] of the leukocyte. The leading edge [?] of the leukocyte inserts
itself between endothelial cells, and the leukocyte migrates through the blood
vessel wall into the inflamed tissue. Rolling, activation, adhesion, and
transendothelio-migration [?] are the foresteps of the process called leukocyte
extravasation [?]. »