After studying this chapter, you should be able to:
Know that many proteins are targeted by signal sequences to their correct destinations and that the Golgi apparatus plays an important role in sorting proteins.
Understand that specialized signals are involved in sorting proteins to mitochondria, the nucleus, and to peroxisomes.
Appreciate that N-terminal signal peptides play a key role in directing newly synthesized proteins into the lumen of the endoplasmic reticulum.
Know that chaperones prevent faulty folding of other proteins, that mechanisms exist for disposing of misfolded proteins, and that the endoplasmic reticulum acts as a quality control compartment.
Explain the role of ubiquitin as a key molecule in protein degradation.
Recognize the important role of transport vesicles in intracellular transport.
Appreciate that many diseases result from mutations in genes encoding proteins involved in intracellular transport and be familiar with the terms conformational diseases and diseases of proteostatic deficiency.
Inside the cell, proteins are synthesized on polyribosomes, but perform their particular functions at many different subcellular locations. Some are destined to be components of specific organelles, others for the cytosol or the various cellular membranes, and yet others are for export. Thus, there is considerable intracellular traffic of proteins. It was first recognized by Blobel in 1970 that for proteins to attain their proper locations, they generally contain information (a signal or coding sequence) that targets them appropriately. This led to the identification of a number of the specific signals (see Table 49–1), and it became apparent that certain diseases result from mutations that affect these signals. In this chapter, we discuss the intracellular traffic of proteins and their sorting and briefly consider some of the disorders that result when abnormalities occur.
TABLE 49–1Sequences or Molecules That Direct Proteins to Specific Organelles |Favorite Table|Download (.pdf) TABLE 49–1 Sequences or Molecules That Direct Proteins to Specific Organelles
|Targeting Sequence or Compound ||Organelle Targeted |
|N-terminal signal peptide ||ER |
|Carboxyl-terminal KDEL sequence (Lys-Asp-Glu-Leu) in ER-resident proteins in COPI vesicles ||Lumen of ER |
|Di-acidic sequences (eg, Asp-X-Glu) in membrane proteins in COPII vesicles ||Golgi membranes |
|Amino terminal sequence (20-50 residues) ||Mitochondrial matrix |
|NLS (eg, Pro2-Lys3-Arg-Lys-Val) ||Nucleus |
|PTS (eg, Ser-Lys-Leu) ||Peroxisome |
|Mannose 6-phosphate ||Lysosome |
MANY PROTEINS ARE TARGETED BY SIGNAL SEQUENCES TO THEIR CORRECT DESTINATIONS
The protein biosynthetic pathways in cells can be considered to be one large sorting system. Many proteins carry signals (usually but not always specific sequences of amino acids) that direct them to their destination, thus ensuring that they are delivered to the appropriate membrane or cell compartment; these signals are a fundamental component of the sorting system. Usually, the signal sequences are recognized and interact with complementary areas of other proteins that serve as receptors for those ...