10.1. Survey of molecular devices found in cells[[Student version, January 17, 2003]] 357
translocationglycosylationdisulfide
bondingdiffusionSFigure 10.3:(Schematic.) Transport of a protein through a pore in a membrane. Outside the cell (right side of
figure), several mechanisms can rectify the diffusive motion of the protein through the pore, for example disulfide
bond formation and attachment of sugar groups (glycosylation). In addition, various chemical asymmetries between
the cell’s interior and exterior environment could enhance chain coiling outside the cell, preventing reentry. These
asymmetries could include differences in pH and ion concentrations. [Adapted from Peskin et al., 1993.]
cytoplasm. Other proteins need to be pushed outside the cell’s outer plasma membrane. Cells
accomplish this “protein translocation” by threading the chain of amino acids through a membrane
pore.
Figure 10.3 shows several mechanisms that can help make translocation a one-way process.
This motor’s “fuel” is the free energy change of the chemical modification the protein undergoes
upon emerging into the extracellular environment. Once the protein is outside the cell, there is no
need for further motor activity: A one-shot motor suffices for translocation.
Polymerization Many cells move not by cranking flagella or waving cilia (Section 5.3.1), but
rather by extruding their bodies in the direction of desired motion. Such extrusions are variously
called pseudopodia, filopodia, or lamellipodia (see Figure 2.11 on page 39). To overcome the
viscous friction opposing such motion, the cell’s interior structure (including its actin cortex; see
Section 2.2.4 on page 48) must push on the cell membrane. To this end, the cell stimulates the
growth of actin filaments at the leading edge. At rest, the individual (ormonomeric)actin subunits
are bound to another small molecule, profilin, which prevents them from sticking to each other.
Changes in intracellular pH trigger dissociation of the actin-profilin complex when the cell needs to
move; the sudden high concentration of actin monomers then avidly assemble to the ends of existing
actin filaments. To confirm that actin polymerization is capable of changing a cell’s shape in this
way,it’s possible to recreate such behaviorin vitro.Asimilar experiment, involving microtubules,
is shown in Figure 10.4: Here the triggered assembly of a just a handful of microtubules suffices
to distend an artificial bilayer membrane.
Actin polymerization can also get coopted by parasitical organisms, including some bacteria.
The most famous of these is the pathogenic bacteriumListeria monocytogenes,which propels itself
through its host cell’s cytoplasm by triggering the polymerization of the cell’s own actin, forming a
bundle behind it. The bundle remains stationary, enmeshed in the rest of the host cell’s cytoskeleton,
so the force of the polymerization motor propels the bacterium forward. Figure 10.5 shows this