Essential Cell Biology 5th edition

134 CHAPTER 4 Protein Structure and Function
(A)
free
subunits
(B)
binding
site
assembled
structures
dimer
helix
Figure 4−25 Identical protein subunits can assemble into complex
structures. (A) A protein with just one binding site can form a dimer
with another identical protein. (B) Identical proteins with two different
binding sites will often form a long, helical filament. (C) If the two
binding sites are positioned appropriately in relation to each other,
the protein subunits will form a closed ring instead of a helix (see also
Figure 4−23B).
(C)
binding
sites
binding
sites
ECB5 04.25
ring
Proteins Can Assemble into Filaments, Sheets, or Spheres
Proteins can form even larger assemblies than those discussed so far.
Most simply, a chain of identical protein molecules can be formed if
the binding site on one protein molecule is complementary to another
region on the surface of another protein molecule of the same type.
Because each protein molecule is bound to its neighbor in an identical
way (see Figure 4−14), the molecules will often be arranged in a helix
that can be extended indefinitely in either direction (Figure 4−25). This
type of arrangement can produce an extended protein filament. An actin
filament, for example, is a long, helical structure formed from many molecules
of the protein actin (Figure 4−26). Actin is extremely abundant
in eukaryotic cells, where it forms one of the major filament systems of
the cytoskeleton (discussed in Chapter 17). Other sets of identical proteins
associate to form tubes, as in the microtubules of the cytoskeleton
(Figure 4−27), or cagelike spherical shells, as in the protein coats of virus
particles (Figure 4−28).
Many large structures, such as viruses and ribosomes, are built from a
mixture of one or more types of protein plus RNA or DNA molecules.
These structures can be isolated in pure form and dissociated into their
constituent macromolecules. It is often possible to mix the isolated components
back together and watch them reassemble spontaneously into
the original structure. This demonstrates that all the information needed
for assembly of the complicated structure is contained in the macromolecules
themselves. Experiments of this type show that much of the
structure of a cell is self-organizing: if the required proteins are produced
in the right amounts, the appropriate structures will form automatically.
Some Types of Proteins Have Elongated Fibrous Shapes
Most of the proteins we have discussed so far are globular proteins, in
which the polypeptide chain folds up into a compact shape like a ball with
an irregular surface. Enzymes, for example, tend to be globular proteins:
even though many are large and complicated, with multiple subunits,
most have a quaternary structure with an overall rounded shape (see
Figure 4−10). In contrast, other proteins have roles in the cell that require
them to span a large distance. These proteins generally have a relatively
simple, elongated three-dimensional structure and are commonly
referred to as fibrous proteins.
Figure 4–26 An actin filament is
composed of identical protein subunits.
(A) Transmission electron micrograph of an
actin filament. (B) The helical array of actin
molecules in an actin filament often contains
thousands of molecules and extends for
micrometers in the cell; 1 micrometer =
1000 nanometers. (A, courtesy of Roger
Craig.)
(A)
(B)
actin molecule
37 nm
50 nm