Signal peptide export signals

PelZ is a hydrophilic protein of 420 amino acids with a short hydrophobic sequence at its N-terminal end which has Ae characteristics of the signal sequences of exported proteins. The signal peptide may be 24 amino acids long, which would corroborate wiA the usual length encountered in prokaryotes. The molecular cloning of the pelZ gene in an expression vector pT7-6 allowed for the specific 35S-cysteine-methionine raAo-labelling of PelZ in E. coli K38. We could detect, in crude extracts, the presence of a precursor and a mature form of PelZ. After cell fractionation, Ae mature form of PelZ could be localized in Ae periplasm of E. coli. So PelZ appears to be a protein exported by Ae Sec-dependent system of translocation. 

Was a common principle found in protein sorting Schatz and Dob-berstein (1996) discussed some similarity in import/export systems between various organelles. It is most interesting to know why apparently vague sequence patterns can be so specific. One key seems to lie in the fact that we have seen several different phenomena as a whole (as in the case of signal peptides). The next 10 years will also be exciting for sequence analysts. 

M. Miiller, Proteolysis in Protein Import and Export Signal Peptide Processing in Eu-and Prokaryotes , Experientia 1992, 48, 118-129. 

Niviere, V., Wong, S. L. and Voordouw, G. (1992) Site-directed mutagenesis of the hydro-genase signal peptide consensus box prevents export of a beta-lactamase fusion protein. J. Gen. Microbiol., 138, 2173-83. 

With all proteins, protein biosynthesis (Translation for details, see p. 250) starts on free ribosomes in the cytoplasm (1). Proteins that are exported out of the cell or into lyso-somes, and membrane proteins of the ER and the plasma membrane, carry a signal peptide for the ER at their N-terminus. This is a section of 15-60 amino acids in which one or two strongly basic residues (Lys, Arg) near the N-terminus are followed by a strongly hydro-phobic sequence of 10-15 residues (see p. 228). 

The ovalbumin gene, shown here, has introns A to G and exons 1 to 7 and L (L encodes a signal peptide sequence that targets the protein for export from the cell see Fig. 27-34). About three-quarters of the... 

The process of protein export involves a small, cytoplasmic ribonucleoprotein particle (the Signal Recognition Particle or SRP) with the signal coding mRNA sequence and/or the signal peptide itself. This interaction stops translation of the protein. Then, the stalled or arrested ribosome moves to the endoplasmic reticulum (ER). A receptor on the ER binds the SRP. 

If export competence is associated with a loosely folded precursor, then parameters that accelerate folding or stabilize folded states should impede export. This relationship has also been explored in E. coli using a DHFR fusion protein. At low levels of synthesis, a hybrid protein consisting of the signal peptide and the first 153 amino acid residues of OmpA joined to DHFR is efficiently secreted. However, addition of trimethoprim imparts a kinetic defect in the secretion rate of the hybrid protein. The effect of trimethoprim is dependent on the presence of a full length, presumably active, DHFR moiety, indicating that secretion in vivo is inhibited by stabilization of the native DHFR structure (FreudI et ai, 1988). 

Hybrid proteins composed of the signal peptide and a substantial portion of the mature MBP fused to LacZ enter the export pathway but are not completely translocated (Bankaitis et ai, 1985 Bassford et al.. 

Throughout this review, terms such as unfolded, loosely folded, or export-competent state are bandied about (no doubt distressing some disciples of protein structure). These terms are not meant to imply a single definable conformation, or the absence of a specific secondary or tertiary structure. In fact, several lines of evidence indicate that a translocation-competent precursor can contain considerable structure. For example, efficient signal peptide function requires that the hydrophobic core assume an a-helical conformation (Jones et ai, 1990), and evidence has been presented that enzymatically active DHFR (Freudl et al., 1988) and a fully folded biotin acceptor domain (Reed and Cronan, 1991)—or conformers of these proteins that are in equilibrium with the native states—can be translocated across the cytoplasmic membrane of E. coli in vivo. 

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