Figure 10.9 illustrates how budding of a vesicle from one organelle, followed by fusion with a second membrane, can transport both soluble proteins and integral membrane proteins to the new compartment. The process retains the "sidedness" of the membrane and the compartment it encloses: the side of an integral membrane protein that faced the lumen of the first compartment ends up facing the lumen of the second compartment, while soluble proteins in the lumen of the first compartment do not enter the cytosol but end up in the lumen of the second compartment, or in the extracellular medium if the target membrane is the plasma membrane.
IN DEPTH 10.1 Trafficking Movies_
Chimeric proteins that contain green fluorescent protein (page 150) and appropriate sorting signals will be processed by the cells machinery just as proteins coded for on its own genes. Thus protein trafficking can be viewed by fluorescence microscopy in live cells that have been transfected with plasmids coding for such chimeras. Movies illustrating many aspects of membrane transport are available on the World Wide Web (see the CBASC website).
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