Signal sequences with proteins

Proteins are targeted to various locations after synthesis by signal sequences. Thus, proteins destined for the ER, the mitochondria and chloroplasts have particular kinds of signal sequences at the N-terminus. ER-targeted proteins enter the ER directly off rough ER ribosomes via a signal recognition particle (SRP) complex that is linked to an SRP receptor and a ribosome receptor-transmembrane peptide translocation complex associated with the ER membrane. Within, the ER polypeptides are processed and folded and S—S links are formed. 

The experiments described in Sections VI,A,B show that two physical properties of the synthetic LamB signal peptides correlate with their in vivo export function tendency to adopt an a-helical conformation in hydrophobic environments, and tendency to insert into lipid mono-layers. These properties may be involved in the same step in the secretion process, or in different steps. An a-helical conformation may be required to generate a structure sufficiently hydrophobic to allow mono-layer insertion. Alternatively, these properties may reflect separate roles of the signal sequence in protein secretion. For instance, an a-helical conformation may be necessary for binding to a proteinaceous site, while the ability to interact with lipids may be important for another step in the secretion process. We have studied the conformations of the synthetic LamB signal peptides in phospholipid vesicles and monolayers by CD and IR spectroscopy. 

Once the synthetic machinery/nascent protein complex has been delivered to the membrane by SRP, it interacts with the membrane to form a translocation-competent species. It is in this process that we envision the signal sequence to play its most active and crucial part. In Section VIII we propose a model for the initial interactions of a signal sequence with the membrane. These interactions bring the signal sequence and nascent protein into the proper orientation for membrane binding and effect the initial entry of the protein into the membrane interior, readying it for the next step, which is translocation. 

In post-translational translocation, a completed secretory protein is targeted to the ER membrane by interaction of the signal sequence with the translocon. The polypeptide chain is then pulled into the ER by a ratcheting mechanism that requires ATP hydrolysis by the chaperone BiP, which stabilizes the entering polypeptide (see Figure 16-9). 

Figure 8.15 Sequence-specific protein-DNA interactions provide a general recognition signal for operator regions in 434 bacteriophage, (a) In this complex between 434 repressor fragment and a synthetic DNA there are two glutamine residues (28 and 29) at the beginning of the recognition helix in the helix-turn-helix motif that provide such interactions with the first three base pairs of the operator region.

Sequence of amino acids that determine the transport of proteins into the nucleus. Although there is no clear consensus, nuclear localization signals tend to be rich in positively charged residues, which allow interaction with proteins from the nuclear import machinery (i.e., importins). 

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. 

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