Biomedical Engineering Reference
In-Depth Information
if energy is constantly fed into the system and this is done by hydrolyzing ATP
into ADP. Phosphorylated monomers polymerize at the barbed end, while de-
phosphorylated monomers depolymerize at the pointed end. In the absence
of any other protein, the polymerization rate of actin is about two orders of
magnitude slower than
in vivo
. To obtain the right values, three other types
of proteins must be added:
a) cofactors, which speed up depolymerization at the pointed end and thus
speeds up turn over and polymerization at the barbed end,
b) capping proteins, which cap free barbed ends and localize polymerization
strictly at the bacterium surface or its immediate neighborhood,
c) a protein complex called Arp2/3 which provides branching to the network
[11, 12], and also speeds up the gel formation.
The identification of the minimum number of constituents necessary to
reproduce
in vitro
, the bacterium motion was an important step towards a
quantitative understanding of the process [13]. Another important step is the
demonstration that it is possible to replace the bacteria itself by inert beads
such as polystyrene beads, on which the enzyme ActA is grafted or adsorbed
[14, 15]. More details on this aspect will be given in Sections 1.4 and 1.5 of
this article.
1.2.2 Elastic Behavior
In the introduction of this article we wrote without further justification that
the comet-like structure of Figure 1.1 was a gel, and that polymerization was
taking place at the bacterium surface. That polymerization is indeed taking
place at the bacterium surface was shown by using fluorescently labeled actin
[16, 17]. This still does not tell us that the actin gel is a real gel in the sense
that it has the mechanical properties of an elastic body. This can be done
by cutting pieces of the comet using laser surgery techniques, and measuring
the bending modulus of the comet [18, 19]. These experiments show that the
comet does have elastic behavior over time scales of minutes. Elastic moduli
are found to be in the kilo-Pascal range with a large total spread of two orders
of magnitude. This spread is not related to a slow depolymerization process
known to exist in the comet and that can be studied independently. These
experiments show that the gel elastic properties must be explicitly taken into
account in the physics of the motion.
Next, one wants to know about the connection between the bacterium sur-
face and the gel. By using laser tweezers or better electric fields one can exert
piconewton forces between bacterium and comet during typically a minute:
no relative motion at a micron resolution can be detected [19]. This shows
that the bacterium is firmly connected to the gel. More quantitatively, if one
describes the bacterium-gel lateral interaction by a friction coecient, such
experiments put a lower limit to the friction coecient, four orders of mag-
