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5.1 The role of the cytoskeleton in intracellular transport

In Section 3 we explained how vesicles bud from donor membranes and fuse with target membranes, which may be quite distant. In such cases, vesicles are actively transported from one site to another, a process that involves motor proteins attached to the vesicle, which propel the vesicle along the cytoskeleton. The microtubule network in particular acts as a trackway for long-range movement of transport vesicles, and this is evident in the movies that show the dual staining of tubulin and secr
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4.5 Summary

  1. Targeting sequences at the N-terminus of proteins direct translation across the ER, and act as signals for import to the nucleus, mitochondrion and chloroplasts. Sequences at the C-terminus control traffic through the ER and the Golgi and to peroxisomes.

  2. Glycosylation is directed by signal sequences that act as targets for N-linked glycosylation in the ER and O-linked glycosylation in the Golgi apparatus. Glycosylation and remodelling of polys
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4.4 Lipidation

Lipid modification occurs in both prokaryotes and eukaryotes, including viruses. Lipidated proteins have an increased affinity for membranes, and the preferential partition of lipid-anchored proteins provides a number of physiological benefits, such as spatial specificity, increased local concentration and faster protein–protein interactions. A number of C-terminal motifs are sites for lipidation, including-CaaX, -CC and –CXC, where X is any amino acid and a is an aliphatic residue.


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4.3 Glycosylation sequences and protein glycosylation

Polysachharide units on proteins may be simple or branched and are almost completely confined to those proteins destined for the cell surface or secretion. The sites and types of glycosylation are determined by the primary structure of the protein and by the availability of enzymes to carry out glycosylation (glycosyltransferases).

N-linked polysaccharides are attached to the –NH2 groups of asparagine and O-linked polysaccharides are attached to the –OH groups of serine a
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4.2 Peptide signal sequences

The distinct chemistry of proteins at the N- and C-termini provides protein molecules with two positionally and chemically unique sites for post-translational modifications and with the means to control their spatial and temporal interactions and position. This feature of proteins is crucial for a variety of biological processes from protein degradation to protein sorting for specific cellular compartments. The N- and C-termini of proteins have distinct roles, and we have already emphasised t
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4.1 Introduction

We have noted how proteins for different destinations are packaged in transport vesicles, a process that depends on signal sequences in the proteins. In this section we shall look in a little more detail at the nature of the signal sequences. Except for the few proteins synthesised in the mitochondria or chloroplasts, cellular proteins are encoded by nuclear genes and synthesised on ribosomes in the cytosol or at the ER. Consequently, these proteins, if destined for organelles, must be sorted
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3.7 Summary

  1. The formation of transport vesicles is initiated by small G proteins that insert into the donor membrane and assemble coat proteins. The coat proteins are COPI, COPII or clathrin, depending on the pathway, and the coat includes adaptor proteins that link the coat to the vesicle and its cargo. Epsins and dynamin are involved in the budding process.

  2. The vesicle cargo depends on the adaptor proteins, sorting proteins and receptors that are assemb
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3.6 Membrane fusion mediated by viral proteins

Until now, we have focused on the transport of material between different intracellular membrane-bound compartments and fusion of cytoplasmic membranes. This type of fusion is endoplasmic fusion. Another type of membrane fusion, called ectoplasmic fusion, is used by enveloped viruses to infect cells (enveloped viruses have an outer phospholipid bilayer). The biophysical and structural studies of viral proteins involved in the processes of membrane fusion provide a foundation for
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3.4 The function of Rab proteins in directing traffic

The SNARE proteins are just one component of the vesicle targeting system. Other participants in this process are the Rab family of GTPases, which regulate traffic between different cellular compartments and which are implicated in directing vesicles to their appropriate target compartments. The Rab family is the largest family of GTPases, with more than 30 members. They are distributed in specific organelles where they mediate the assembly of distinctive groups of proteins. Moreover,
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3.3 Fusion of vesicles with the target membrane

In this section, we shall look at how vesicles fuse with the appropriate target membrane. The targeting of different classes of transport vesicles to their distinct membrane destinations is essential in maintaining the distinct characteristics of the various eukaryotic organelles. Because coat proteins, such as clathrin, are found in different trafficking pathways, it follows that other proteins in the coat must specify the direction of transport of a particular vesicle and its ultimate desti
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3.1 Introduction

In the following sections, we shall describe the sequential steps involved in the movement of vesicles from one membrane to another (see Figure 9). Some of these steps are quite well defined, but for others there are gaps in our knowledge. Although we have emphasised the importance of proteins as cargo, vesicles also transfer membra
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2.7 Summary

  1. Eukaryotic cells contain numerous distinct types of membrane-bound compartment. Transport vesicles move proteins and other molecules between the compartments.

  2. Proteins contain signalling sequences or patches that specify their destination compartment.

  3. Proteins destined for lysosomes, secretion or the plasma membrane are synthesised in the ER, transported to the cis Golgi, modified in the Golgi apparatus, and sorted and pa
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1.3 Polymerisation and depolymerisation of tubulin

Polymerisation of microtubules is similar in concept to microfilament polymerisation, but different in almost every detail. The basic subunit of the microtubule is the tubulin heterodimer, consisting of an α-tubulin and a β-tubulin monomer, which are firmly associated with each other. These assemble end-to-end to form filaments. The overall assembly consists of a ring of 13 such filaments arranged into a microtubule with a plus and a minus end (Author(s): The Open University

4.2 Summary

  1. Glycogen metabolism is controlled by two enzymes, glycogen synthase (mediating glycogen synthesis) and phosphorylase (mediating glycogen breakdown).

  2. Three pathways converge in the regulation of glycogen synthase: cAMP/PKA and GSK-3β are negative regulators, whereas ISPK/PP1G positively regulate the activity of glycogen synthase.

  3. Insulin and adrenalin have opposite effects on glycogen synthesis: insulin promotes glycogen synthes
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4.1 Glucose metabolism

We are now in a position to draw together the major concepts and components of signalling, and show how they operate in one well-understood system, namely the regulation of the storage or release of glucose in the human body. From this, you will be able to recognize archetypal pathways represented in specific examples, you will be able to appreciate how the same basic pathways can be stimulated by different hormones in different tissues, and you will see how opposing hormones activate separat
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3.3.2 Phospholipase C (PLC)

Members of this family of enzymes contain two catalytic domains and several protein binding domains (Figure 13). The PH domain can temporarily tether phospholipase C to the membrane by attachment mainly to PI(3,4)P2.

We shall discuss two main isoforms of PLC: PLC-β, which is activated by a subset of trimeric G proteins (Gαq an
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2.1 Introduction

Every receptor has to be able to recognize its particular ligand in a specific manner, and become activated by it in such a way that it transmits the signal to the cell. We shall deal with receptor specificity and activation mechanisms. Then we shall see how the same principles of specificity and activation also apply to intracellular receptors.


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5.1 Introduction

In some ways, proteins can be thought of as molecular machines, that through evolution have become highly specialised and efficient. Despite the somewhat static representations of proteins that you have met so far, proteins are in fact dynamic molecules. Not only are there internal movements, with conformational changes that are integral to protein function and regulation of function, but proteins, by virtue of their specific interactions with other cellular components, are essential to all t
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4.5 Summary of Section 4

  1. Proteins that serve similar functions often have similar amino acid sequences. The sequence of a protein can point to a particular function and can also be used to predict protein conformation.

  2. Homologous proteins contain statistically significant (i.e. above random) similarities in their amino acid sequences.

  3. The SH2 domain is an example of a domain that is highly conserved across different eukaryotes and is common to many diff
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2.4.2 Lipid-linked proteins and lipoproteins

Lipid-linked proteins are proteins that have been covalently modified by addition of one or more lipid groups. Note that the term lipoprotein, though sometimes used to describe lipid-linked proteins, is strictly applicable only to those proteins that associate with lipids non-covalently. These proteins have quite distinct functions. Lipoproteins serve to transport triacylglycerols and cholesterol in the blood plasma. We will not be discussing them any further at this point.

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