After studying this chapter, you should be able to:
Indicate that many proteins are targeted by signal sequences to their correct destinations and that the Golgi apparatus plays an important role in sorting proteins.
Recognize that specialized signals are involved in sorting proteins to mitochondria, the nucleus, and to peroxisomes.
Explain that N-terminal signal peptides play a key role in directing newly synthesized proteins into the lumen of the endoplasmic reticulum (ER).
Explain how chaperones prevent faulty folding of other proteins, how misfolded proteins are disposed of, and how the ER 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.
Indicate that many diseases result from mutations in genes encoding proteins involved in intracellular transport.
Proteins are synthesized on polyribosomes, but perform varied functions at different subcellular locations, including the cytosol, specific organelles, or membranes.Yet others are destined for export. Thus, there is considerable intracellular traffic of proteins. As recognized by Blobel in 1970, to enable proteins to attain their proper locations, their structure contains a signal or coding sequence that targets them appropriately. This led to the identification of numerous specific signals (Table 49–1), and also the recognition that certain diseases result from mutations that adversely affect these signals. This chapter will discuss the sorting and intracellular traffic of proteins 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 |
|Diacidic 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 specific subcellular destinations; 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 which recognize the signals.
A major sorting decision is made early in protein biosynthesis, when specific proteins are synthesized either on cytosolic (free) or membrane-bound polyribosomes (see Chapter 37). The signal hypothesis was proposed by Blobel and Sabatini in 1971 partly to explain the distinction ...