Signal sequences

Expoitins are transport receptors at the nuclear pore complex needed for the selective export of proteins from the nucleus into the cytoplasm. They recognize nuclear export signal sequences of cargo proteins. 

GPI anchoring is a posttranslational modification occurring in the endoplasmic reticulum where preassembled GPI anchor precursors are transferred to proteins bearing a C-terminal GPI signal sequence. The GPI anchor precursors are synthesized in the endoplasmic reticulum by sequential addition of sugar and other components to phosphatidylinositol. Protein GPI anchors are ubiquitous in eukaryotic cells. In mammalian cells, GPI anchored proteins are often found in lipid rafts which are subdomains of the plasma membrane, containing various signaling components. 

CNTF is expressed in glial cells within the central and peripheral nervous system. CNTF lacks a signal sequence and is not secreted by the classical secretory pathway, but is thought to convey its cytoprotective effects after release from adult glial cells by some mechanism induced by injury [3,5]. 

The signal recognition particle (SRP) is a cytosolic ribonucleoprotein complex which binds to signal sequences of nascent membrane and secretory proteins emerging from ribosomes. The SRP consists of a 7S RNA and at least six polypeptide subunits (relative molecular masses 9, 14, 19, 54, 68, and 72 kD). It induces an elongation arrest until the nascent chain/ ribosome/SRP complex reaches the translocon at the endoplasmic reticulum (ER) membrane. 

Fig. 18 CG15920 gene sequence and primary structure. The consensus repeat sequences are also represented. The highlighted regions correspond to the signal sequence, R R chitin-binding domain, and the elastomeric domains containing repeat motifs A and B. Reproduced from [182, 188] with permission from Elsevier, copyright Elsevier 2001, 2010...

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

The above describes the major pathway of proteins destined for the mitochondrial matrix. However, certain proteins insert into the outer mitochoiidrial membrane facilitated by the TOM complex. Others stop in the intermembrane space, and some insert into the inner membrane. Yet others proceed into the matrix and then return to the inner membrane or intermembrane space. A number of proteins contain two signaling sequences—one to enter the mitochondrial matrix and the other to mediate subsequent relocation (eg, into the inner membrane). Certain mitochondrial proteins do not contain presequences (eg, cytochrome Cy which locates in the inter membrane space), and others contain internal presequences. Overall, proteins employ a variety of mechanisms and routes to attain their final destinations in mitochondria. 

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. 

E. coli strain BL21(DE3) (F ompT ksdSafra ms ) gal dcm) (Novagen, Madison, WI, USA) was used as a host for recombinant OPH expression. Recombinant plasmids pTOH and pEOH that contain OPH gene fused with hexa-histidine affinity tag under trc and T7 promoter, respectively, as control vectors and pTTOH and pETOH that contain OPH gene fused with Tat signal sequence and hexa-histidine affinity tag under trc and T7 promoter, respectively, were used (Fig. 1). 

Fig.l. Gene maps of recombinant plasmids pTOH, pTTOH, pEOH, and pETOH. Abbreviations Pnc, trc promoter Px7, T7 and lac hybrid promoter Tat, twin-arginme TorA signal sequence of TMAO reductase OPH, organophosphoms hydrolase gene Hisg, hexa-histidine affinity tag. 

Signal sequence Amino acid sequence in protein, whose function is to direct its final intracellular or exh acellular location. 

Upper panel The hydropathy profile of the entire 69 kD precursor protein is shown. The abscissa is amino acid residues and the ordinate, positive values indicate hydrophilic. The black and hatched rectangles at the bottom of the figure denote the calculated signal sequence and amino-terminal propeptide domains, respectively. The mature and carboxyl-terminal domains are labeled. N-linked core glycosylation consensus sites are depicted by branched structures. 

The PelX protein contains 749 amino-acids including an amino-terminal signal sequence of 26 amino-adds. The celliilar localisation of the enzyme has not been rigorously determined. In E. chrysanthemi CUCPB1237, the exo-pectate lyase activity was cell-bound (14). The presence of this type of enzyme in the bacterial periplasm appeared normal since it acts better on oligomers produced by endo-pectate lyases than on long polymeric substrates (15). 

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