Protein Signal sequence Targeting

A Hydrophobic N-Terminal Signal Sequence Targets Nascent Secretory Proteins to the ER... 

MANY PROTEINS ARE TARGETED BY SIGNAL SEQUENCES TO THEIR CORRECT DESTINATIONS... 

Many proteins are targeted to their destinations by signal sequences. A major sorting decision is made when proteins are partitioned between cytosohc and membrane-bound polyribosomes by virtue of the absence or presence of a signal peptide. 

Walter, P., andjohnson, A. E. (1994). Signal sequence recognition and protein targeting to the endoplasmic reticulum membrane. Annu. Rev. Cell Biol. 10, 87—119. 

Answer B. Decreased Factor V secretion and a corresponding accumulation of cytoplasmic antigen suggest a defect in the translocation of the nascent protein to the endoplasmic reticulum. This implies a mutation in the N-terminal amino acid signal sequence required for targeting to the ER and encoded by the first exon of the gene. 

Kitada S, Uchiyama T, Funatsu T, Kitada Y, Ogishima T, Ito A (2007) A protein from a parasitic microorganism, Rickettsia prowazekii, can cleave the signal sequences of proteins targeting mitochondria. J Bacteriol 189 844-850... 

The most important element in many of these targeting pathways is a short sequence of amino acids called a signal sequence, whose function was first postulated by Gunter Blobel and colleagues in 1970. The signal sequence directs a protein to its appropriate location in the cell and, for many proteins, is removed (luring transport or after the protein has reached its final destination. In proteins slated for transport into mitochondria, chloroplasts, or the ER, the signal sequence... 

In most multicellular eukaryotes, the nuclear envelope breaks down at each cell division, and once division is completed and the nuclear envelope reestablished, the dispersed nuclear proteins must be reimported. To allow this repeated nuclear importation, the signal sequence that targets a protein to the nucleus—the nuclear localization sequence, NLS—is not removed after the protein arrives at its destination. An NLS, unlike other signal sequences, may be located almost anywhere along the primary sequence of the protein. NLSs can vary considerably, but many consist of four to eight amino acid residues and include several consecutive basic (Arg or Lys) residues. 

Bacteria Also Use Signal Sequences for Protein Targeting... 

Bacteria can target proteins to their inner or outer membranes, to the periplasmic space between these membranes, or to the extracellular medium. They use signal sequences at the amino terminus of the proteins (Fig. 

FIGURE 27-38 Signal sequences that target proteins to different locations in bacteria. Basic amino acids (blue) near the amino terminus and hydrophobic core amino acids (yellow) are highlighted. The cleavage sites marking the ends of the signal sequences are indicated... 

After synthesis, many proteins are directed to particular locations in the cell. One targeting mechanism involves a peptide signal sequence,... 

Proteins targeted to mitochondria and chloroplasts in eukaryotic cells, and those destined for export in bacteria, also make use of an amino-terminal signal sequence. 

Proteins targeted to the nucleus have an internal signal sequence that is not cleaved once the protein is successfully targeted. 

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