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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4.2 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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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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2.5 The endocytic pathways and lysosomes

Endocytosis is the process by which cells internalise molecules from the outside, and it includes pinocytosis, the uptake of small soluble molecules in vesicles, and phagocytosis, the internalisation of large insoluble particles. These are two ends of a spectrum as seen microscopically, but the receptors, the subsequent intracellular trafficking pathways and the fate of the internalised molecules, vary depending on the cell type and its functions. The endocytic pathway co
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2.4 Exocytosis and the secretory pathways

Exocytosis is the process by which molecules are released to the outside of the cell. This includes the release of proteins to the plasma membrane and the release of secreted molecules into the extracellular fluid. All eukaryotic cells need a system to transport molecules to their plasma membrane, and many cells secrete proteins into the extracellular environment. In addition, cells in multicellular organisms communicate with each other via a variety of signalling molecules, which are
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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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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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5.3.3 Phosphorylation of proteins as a means of regulating activity

Phosphorylation is an important mechanism for regulating the activity of many proteins, either switching on or switching off some activity of the protein.

  • What protein that we have already discussed is both positively and negatively regulated by phosphorylation?

  • Src kinase activity is switched on by dephosphorylation of
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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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1.6 Fibrous proteins

Most of the proteins described so far have been globular proteins. There are, however, some distinctive features that characterise fibrous proteins and we present here a general overview of these. Elongated fibrous proteins frequently play a structural role in the cell. They do not readily crystallise but tend to aggregate along their long axis to form fibres. X-ray diffraction studies of these fibres, in contrast to analysis of protein crystals, provides only very limited information on the
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3 Reproduction in marsupials

The study of mammals requires you to deal with measurements, which we call numerical ‘data’, and you will get practice with compiling and analysing data if you work through all the units in this series. We assume only that you can add, subtract, multiply and divide. In this section, we ask you to use units
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1.4.5 Velocity–time and speed–time graphs

Just as we may plot the position–time graph or the displacement–time graph of a particular motion, so we may plot a velocity–time graph for that motion. By convention, velocity is plotted on the vertical axis (since velocity is the dependent variable) and time (the independent variable) is plotted on the horizontal axis. In the special case of uniform motion, the velocity–time graph takes a particularly simple form – it is just a horizontal line, i.e. the gradient is zero. Ex
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5 Summary

Domesticated organisms evolve in artificial environments under artificial selection, and opportunistic or enforced hybridisation often occurs between species that would not normally interbreed. Natural selection cannot be eliminated and continues to operate. At least two different forms of dwarfism are common in domesticated livestock and humans, but only the rarer midget type of dwarfism occurs in wild lineages. Domesticated mammals and birds have distinctive patterns of skin pigmentation th
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2.2 Size and shape

The shape of the head is determined mainly by the relative sizes of the jaws and the nose and the back of the skull containing the brain, eyes, ears and, in artiodactyls, the horns or antlers. All these structures may differ greatly between otherwise similar species.

SAQ 7


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3.4 Competition

In plants it is particularly obvious that many more potential offspring (seeds) are produced than can survive. To a very large extent it is a matter of chance as to which are the survivors. Some are eaten, others overlooked or stored away and forgotten. Those that survive to germinate might be on unsuitable soil, too dry or too wet, so that they shrivel or rot. The successful seedling could be in poor soil, deficient in minerals, or there may be many other plants that are already established
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2.6 The surface

David A. Rothery Teach Yourself Planets, Chapter 6, pp. 66–75, Hodder Education, 2000, 2003.

Copyright © David Rothery

Look at the Moon even with the unaided eye, and you will see that it has dark patches on a paler background (Figure 2). This simple observation picks out the two distinct types of crust on the Moon. The paler areas are the lunar highlands, and the darker areas are the lunar ‘seas’ or maria (singular: mare). Both the highla
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2.4 The atmosphere and polar ice

David A. Rothery Teach Yourself Planets, Chapter 6, pp. 66–75, Hodder Education, 2000, 2003.

Copyright © David Rothery

The Moon's atmosphere is almost as insubstantial as Mercury's, and probably has much the same origin. The Clementine mission returned our first clear views of the lunar poles, showing sites in particular near the south pole that are permanently in shadow, and which could therefore be places where ice might accumulate (Figure 1). Clementine'
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