Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

128 Chapter 3: Proteins
subunit
hexagonally
packed
sheet
tube
Figure 3–26 Single protein subunits form
protein assemblies that feature multiple
protein–protein contacts. Hexagonally
packed globular protein subunits are
shown here forming either flat sheets or
tubes. Generally, such large structures are
not considered to be single “molecules.”
Instead, like the actin filament described
previously, they are viewed as assemblies
formed of many different molecules.
Some protein subunits assemble into flat sheets in which the subunits are
arranged in hexagonal patterns. Specialized membrane proteins are sometimes
arranged this way in lipid bilayers. With a slight change in the geometry of the
individual subunits, a hexagonal sheet can be converted into a tube (Figure 3–26)
or, with more changes, into a hollow sphere. Protein tubes and spheres that bind
specific RNA and DNA molecules in their interior form the coats of viruses.
The formation of closed structures, such as rings, tubes, or spheres, provides
additional stability because it increases the number of bonds between the protein
MBoC6 m3.29/3.25
subunits. Moreover, because such a structure is created by mutually dependent,
cooperative interactions between subunits, a relatively small change that affects
each subunit individually can cause the structure to assemble or disassemble.
These principles are dramatically illustrated in the protein coat or capsid of many
simple viruses, which takes the form of a hollow sphere based on an icosahedron
(Figure 3–27). Capsids are often made of hundreds of identical protein subunits
that enclose and protect the viral nucleic acid (Figure 3–28). The protein in such
a capsid must have a particularly adaptable structure: not only must it make
several different kinds of contacts to create the sphere, it must also change this
arrangement to let the nucleic acid out to initiate viral replication once the virus
has entered a cell.
Many Structures in Cells Are Capable of Self-Assembly
The information for forming many of the complex assemblies of macromolecules
in cells must be contained in the subunits themselves, because purified subunits
can spontaneously assemble into the final structure under the appropriate conditions.
The first large macromolecular aggregate shown to be capable of self-assembly
from its component parts was tobacco mosaic virus (TMV ). This virus is
a long rod in which a cylinder of protein is arranged around a helical RNA core
(Figure 3–29). If the dissociated RNA and protein subunits are mixed together in
solution, they recombine to form fully active viral particles. The assembly process
is unexpectedly complex and includes the formation of double rings of protein,
which serve as intermediates that add to the growing viral coat.
Another complex macromolecular aggregate that can reassemble from its
component parts is the bacterial ribosome. This structure is composed of about
55 different protein molecules and 3 different rRNA molecules. Incubating a mixture
of the individual components under appropriate conditions in a test tube
causes them to spontaneously re-form the original structure. Most importantly,
such reconstituted ribosomes are able to catalyze protein synthesis. As might be
expected, the reassembly of ribosomes follows a specific pathway: after certain
proteins have bound to the RNA, this complex is then recognized by other proteins,
and so on, until the structure is complete.
It is still not clear how some of the more elaborate self-assembly processes
are regulated. Many structures in the cell, for example, seem to have a precisely
defined length that is many times greater than that of their component macromolecules.
How such length determination is achieved is in many cases a mystery. In
20 nm
Figure 3–27 The protein capsid of a
virus. The structure of the simian virus
SV40 capsid has been determined by x-ray
crystallography MBoC6 and, as m3.30/3.26 for the capsids of
many other viruses, it is known in atomic
detail. (Courtesy of Robert Grant, Stephan
Crainic, and James M. Hogle.)