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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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2.3 Sorting for the basolateral and apical zones of the plasma membrane

Many cells are permanently polarised, and this means that surface proteins are selectively localised to different areas of the plasma membrane, depending on their function. For example, endothelial cells have adhesion molecules on the surface that contacts the basal lamina, but receptors that take up molecules from the blood (e.g. the transferrin receptor – see below) are located on the surface of the cell that is in contact with the blood. Cell surface molecules can normally diffuse latera
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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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3.9 Summary

  1. Heterotrimeric G proteins are tethered to the internal surface of the plasma membrane, and are activated by conformational change within 7TM receptors. There are many different α subunits (and a few βγ subunits), which interact with different receptors and different effectors. The major targets of G proteins include ion channels, adenylyl cyclase (activated by Gαs and inhibited by Gαi) and PLC-β (activated by Gαq).


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3.4.2 Cyclic AMP

The concentration of cyclic AMP (cAMP) in the cytosol increases 20-fold within seconds of an appropriate stimulus. This is achieved by the action of the plasma membrane-embedded protein adenylyl cyclase, which synthesizes cAMP from ATP (Figure 34). cAMP is short-lived, as with all second messengers, because it is continuously degraded by
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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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Acknowledgements

Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

The content acknowledged below is Proprietary and used under licence (not subject to Creative Commons licence). See Terms and Conditions.

Figures

Figures 3, 5–7, 40, 41 Voet, D. and Voet, J. G. (1995) Biochemistry, 2nd edn, copyright © 1995 John Wiley & Sons Inc

Figures 4, 8, 9a, 10, 14, 24, 25a,c Voet, D. and Voet,
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7.3.1 Physical methods for demonstrating an interaction between proteins

To identify those unknown proteins in a complex mixture that interact with a particular protein of interest, protein affinity chromatography can be used (Figure 49a). This approach uses a ‘bait’ protein attached to a matrix. When this baited matrix material is then exposed to a mixture of proteins, only proteins that interact with the
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6.6 Summary of Section 6

  1. The majority of proteins of known function are enzymes. Enzymes are biological catalysts, increasing the rates of reactions. Enzymes are not permanently altered by catalysis of a reaction.

  2. The transition state is an unstable intermediate enzyme–substrate complex in which the enzyme and the substrate are in highly strained conformations.

  3. There are a number of different catalytic mechanisms employed by enzymes including general
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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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4.2 Amino acid sequence homologies and why they occur

Consider two genes encoding proteins that have 50% of their amino acid sequence in common.

  • How can this sequence homology be explained in terms of evolution?

  • The most parsimonious explanation is that the similarities result from the fact that the two organisms share a common evolutionary past and that the genes encoding
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3.5 Summary of Section 3

  1. Protein domains allow segregation of different functions in the same protein. They can have a binding function, a structural function or a catalytic function.

  2. Binding domains mediate interactions between proteins of related function (such as those in a signalling cascade) and often are important in regulation of activity. Interactions via these binding domains are often dependent on the phosphorylation state of one of the binding partners. Exa
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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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2.4.1 Glycosylation

Glycosylation of a protein entails the covalent attachment of carbohydrate groups (typically oligosaccharides) and the resulting modified protein is called a glycoprotein. Covalent attachment of sugar residues to proteins occurs in the endoplasmic reticulum (ER) and Golgi apparatus. The oligosaccharide chains usually contain less than 15 sugar residues but are very diverse and are often branched. They are linked to the protein component via either the –OH groups of serine and threoni
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2.4 The covalent modification of proteins

Many proteins are modified by the covalent linking of groups that can affect their function and/or localisation in the cell. Such covalent modifications occur after synthesis and folding of the polypeptide component. The main types of covalent modification and their functions are listed below.

  1. Methylation/acetylation of amino acids at the N-terminal tails of histone proteins in eukaryotes can affect the structure of chromatin and ultimately gene
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