Biology Reference
In-Depth Information
may be determined in part by the mix of phospholipid shapes present but, in living cells, new
membranes form from existing ones and never form spontaneously from their separated
components. In eukaryotic cells, for example, new phospholipids are inserted into the
membrane of the endoplasmic reticulum by enzymes that are already associated with that
membrane, and the new components reach other parts of the cell as part of the bilayer of traf-
ficking vesicles.
11,12
This is another illustration of the point made in Chapter 2, that a given
form may be produced in several ways so that a researcher cannot deduce a morphogenetic
mechanism solely from its result.
ONE-DIMENSIONAL SELF-ASSEMBLY: ACTIN
Actin is the chief constituent of microfilaments, an important cytoskeletal system that
will be described in more detail in Chapters 5, 8 and others. Actin monomers are proteins
that consist of a single peptide chain folded into two lobes, between which there is a deep
cleft occupied by a single molecule of ADP or ATP. The monomers can polymerize to form
a twisting filament two subunits wide, in which each subunit makes contact with four
other subunits. A solution of actin monomers in an aqueous, Mg
2
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-containing buffer
will produce filaments by spontaneous self-assembly as long as the monomer concentra-
tion is above the critical concentration (about 0.1
M for ATP-actin). The dynamics of
the process are not simple, however, and follow a sigmoidal plot in which polymerization
becomes rapid only after a time lag. This lag reflects the fact that actin polymerization is to
some extent 'autocatalytic': the probability of monomers being added to an existing fila-
ment is significantly higher than the formation of filaments
de novo
.Actindimers,the
smallest possible multimers, are highly unstable and tend to fall apart before additional
momoners can be added. Trimers, when they do eventually occur, are much more stable
and are therefore able to nucleate the formation of stable filaments, which grow as new
monomers locate their ends. The lag period
in vitro
is a result of the low probability of
producing trimers.
Cells, and even some infectious bacteria that grow within them, use the ability of actin to
self-assemble into filaments but control it using: (1) proteins that sequester monomers to
prevent inappropriate polymerization; (2) proteins that encourage nucleation when the
formation of new filaments is wanted; and (3) proteins that bind to the ends of filaments
to block their growth or to block their destruction. This is typical of how self-assembly is
used in real morphogenesis: a self-assembling process is embedded in the wider context of
regulatory proteins that control the self-assembly in response to other factors and therefore
direct it to serve higher levels of organization.
m
O
NE-DIMENSIONAL SELF-ASSEMBLY: COLLAGE
N
Another example of biological self-assembly that has been the subject of much attention
over the last 30 years or so is that of the extracellular matrix protein, collagen. Collagen
proteins assemble by associating into fibrils. Many other molecules associate with collagen
and are required for its function but they are not required for the specific act of fibril
