Eukaryotic Cells Contain Compartments Bounded by Internal Membranes

I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.6. Lipids and Many Membrane Proteins Diffuse Rapidly in the Plane of the Membrane
Figure 12.34. Asymmetry of the Na+ -K+ transport system in plasma membranes. The Na+-K+ transport system
pumps Na+ out of the cell and K+ into the cell.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.7. Eukaryotic Cells Contain Compartments Bounded by Internal Membranes
Thus far we have considered only the plasma membrane of cells. Many bacteria such as E. coli have two membranes
separated by a cell wall (made of proteins, peptides, and carbohydrates) lying in between (Figure 12.35). The inner
membrane acts as the permeability barrier, and the outer membrane and the cell wall provide additional protection. The
outer membrane is quite permeable to small molecules owing to the presence of porins. The region between the two
membranes containing the cell wall is called the periplasm. Other bacteria and archaea have only a single membrane
surrounded by a cell wall.
Eukaryotic cells, with the exception of plant cells, do not have cell walls, and their cell membranes consist of a single
lipid bilayer. In plant cells, the cell wall is on the outside of the plasma membrane. Eukaryotic cells are distinguished by
the use of membranes inside the cell to form internal compartments (Figure 12.36). For example, peroxisomes,
organelles that play a major role in the oxidation of fatty acids for energy conversion, are defined by a single membrane.
Mitochondria, the organelles in which ATP is synthesized, are surrounded by two membranes. Much like the case for a
bacterium, the outer membrane is quite permeable to small molecules, whereas the inner membrane is not. Indeed,
considerable evidence now indicates that mitochondria evolved from bacteria by endosymbiosis (Section 18.1.2). A
double membrane also surrounds the nucleus. However, the nuclear envelope is not continuous but, instead, consists of a
set of closed membranes that come together at structures called nuclear pores. These pores regulate transport into and
out of the nucleus. The nuclear membranes are linked to another membrane-defined structure, the endoplasmic
reticulum, which plays a host of cellular roles, including drug detoxification and the modification of proteins for
secretion (Section 11.3.4). Thus, a eukaryotic cell comprises interacting compartments, and transport into and out of
these compartments is essential to many biochemical processes.
12.7.1. Proteins Are Targeted to Specific Compartments by Signal Sequences
The compartmentalization of eukaryotic cells makes possible many processes that must be separated from the remainder
of the cellular environment to function properly. Specific proteins are found in peroxisomes, others in mitochondria, and
still others in the nucleus. How do proteins end up in the proper compartment? Even for bacteria, some targeting of
proteins is required: some proteins are secreted from the cell, whereas others remain in the cytosol.
Proteins include specific sequences that serve as address labels to direct the molecules to the proper location. For
example, most peroxisomal proteins end with a sequence, Ser-Lys-Leu-COO- , that acts as an autonomous targeting
signal. The removal of this sequence from a protein that normally resides in peroxisomes blocks its import into that
organelle, whereas the addition of this sequence to a protein that normally resides in the cytosol can direct that protein to
peroxisomes. A protein destined to pass through both mitochondrial membranes usually has a targeting sequence at its
amino terminus (Figure 12.37). Unlike the peroxisomal targeting sequence, these amino-terminal sequences are highly
variable; no clear consensus exists. They are typically from 15 to 35 residues long and rich in positively charged residues
and in serines and threonines. Proteins destined for the nucleus have internal targeting sequences. A typical nuclear
localization signal contains five consecutive positively charged residues such as Lys-Lys-Lys-Arg-Lys. The addition of
such a sequence to a protein not found in the nucleus can direct it to the nucleus (Figure 12.38). Other sequences can
direct proteins out of the nucleus. The known targeting sequences are given in Table 12.4.
Targeting sequences act by binding to specific proteins associated with each organelle. The determination of the
structure of a protein, α-karyopherin, that binds to the nuclear localization signal reveals how the protein recognizes
such a targeting sequence (Figure 12.39). A peptide containing the appropriate sequence binds to a specific site on the
protein. The target peptide is held in an extended conformation through interactions between the target peptide backbone
and asparagine side chains of the α-karyopherin while each of the basic residues lies in a deep pocket near the bottom,
lined with negatively charged residues. Proteins that bind to the other targeting signal sequences presumably also have
structures that allow recognition of the required features. Note that we have considered only how proteins are marked for
different compartments. Later, we will consider the mechanisms by which proteins actually cross membranes (Section
11.3.2).
12.7.2. Membrane Budding and Fusion Underlie Several Important Biological
Processes
Membranes must be able to separate or join together to take up, transport, and release molecules. Many take up
molecules through the process of receptor- mediated endocytosis (Figure 12.40). Here, a protein or larger complex
initially binds to a receptor on the cell surface. After the protein is bound, specialized proteins act to cause the membrane
in the vicinity of the bound protein to invaginate. The invaginated membrane eventually breaks off and fuses to form a
vesicle.
Receptor-mediated endocytosis plays a key role in cholesterol metabolism (Section 26.3.3). Some cholesterol in the
blood is in the form of a lipid-protein complex called low-density lipoprotein (LDL). Low density lipoprotein binds to an
LDL receptor, an integral membrane protein. The segment of the plasma membrane containing the LDL-LDL receptor
complex then invaginates and buds off from the membrane. The LDL separates from the receptor, which is recycled
back to the membrane in a separate vesicle. The vesicle containing the LDL fuses with a lysosome, an organelle
containing an array of digestive enzymes. The cholesterol is released into the cell for storage or use in membrane
biosynthesis, and the remaining protein components are degraded. Various hormones, transport proteins, and antibodies
employ receptormediated endocytosis to gain entry into a cell. A less advantageous consequence is that this pathway is
available to viruses and toxins as a means of entry into cells. The reverse process the fusion of a vesicle to a
membrane is a key step in the release of neurotransmitters from a neuron into the synaptic cleft (Figure 12.41).
Although the processes of budding and fusion appear deceptively simple, the structures of the intermediates in the
budding and fusing processes and the detailed mechanisms remain active areas of investigation.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.7. Eukaryotic Cells Contain Compartments Bounded by Internal Membranes
Figure 12.35. Cell Membranes of Prokaryotes. A schematic view of the membrane in bacterial cells surrounded by
(A) two membranes or (B) one membrane.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.7. Eukaryotic Cells Contain Compartments Bounded by Internal Membranes
Figure 12.36. Internal Membranes of Eukaryotes. Electron micrograph of a thin section of a hormone-secreting cell
for the rat pituitary, showing the presence of internal structures bounded by membranes. [Biophoto Associates/Photo
Researchers.]
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.7. Eukaryotic Cells Contain Compartments Bounded by Internal Membranes
Figure 12.37. A Mitochondrial Targeting Sequence. This sequence is recognized by receptors on the external face of
the outer mitochondrial membrane. A protein bearing the sequence will be imported into the mitochondrion.
Hydrophobic residues are shown in yellow, basic ones in blue, and serine and threonine in red.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.7. Eukaryotic Cells Contain Compartments Bounded by Internal Membranes
Figure 12.38. Movement of a Protein Into the Nucleus. Localization of (A) unmodified pyruvate kinase, and (B)
pyruvate kinase containing a nuclear localization signal sequence attached to its amino terminus. The protein was
visualized by fluorescence microscopy after staining with a specific antibody. [From W. D. Richardson, B. L. Roberts,
and A. E. Smith. Cell 44(1986):79.]
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.7. Eukaryotic Cells Contain Compartments Bounded by Internal Membranes
Table 12.4. Targeting sequences
Target
Signal
Nucleus
-KKXK or -(K/R)2-X10 12-(K/R)*
Peroxisome
-SKL-COON-terminal amphipathic helix
Mitochondrion
Endoplasmic reticulum -KDEL-COO-(ER retention)
*
The "/" means that either K or R is required.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.7. Eukaryotic Cells Contain Compartments Bounded by Internal Membranes
Figure 12.39. Protein Targeting Signal Recognition. The structure of the nuclear localization signal-binding protein αkaryopherin (also known as α-importin) with a nuclear localization signal peptide bound to its major recognition
site.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.7. Eukaryotic Cells Contain Compartments Bounded by Internal Membranes
Figure 12.40. Receptor-Mediated Endocytosis. The process of receptor-mediated endocytosis is illustrated for the
cholesterol-carrying complex, low-density lipoprotein (LDL): (1) LDL binds to a specific receptor, the LDL receptor; (2)
this complex invaginates to form an internal vesicle; (3) after separation from its receptor, the LDL-containing vesicle
fuses with a lysosome, leading to degradation of the LDL and release of the cholesterol.
I. The Molecular Design of Life
12. Lipids and Cell Membranes
12.7. Eukaryotic Cells Contain Compartments Bounded by Internal Membranes
Figure 12.41. Neurotransmitter Release. Neurotransmitter-containing synaptic vesicles are arrayed near the plasma
membrane of a nerve cell. Synaptic vesicles fuse with the plasma membrane, releasing the neurotransmitter into the
synaptic cleft. [T. Reese/Don Fawcett/ Photo Researchers.]