Hydrophobicity, signal peptides

The sequences of halocyanin and sulfocyanin, deduced from their gene sequences, have unusually long N-terminal extensions. These extensions feature a hydrophobic signal peptide which is followed by a segment that in the case of halocyanin has an Asn-Gly doublet occurring consecutively seven times. In sulfocyanin, this segment is rich in Ser residues. It is believed that these extensions are covalently modified by a lipid moiety which anchors the proteins to the cell membrane. 

A study with the hydrophobic signal peptide of coli lambda phage in phospholipid monolayers, showed a preference for pC helical conformation when the peptide was inserted into the lipid phase (18), However, interaction with the lipid surface without insertion induced the peptide to adopt the -structure ), These observations, obtained with circular dichroism and Fourier transform-infrared (FT-IR) data, provide the first direct evidence for interconversions between various conformational... 

Fig. 1 Primary structure of human tropoelastin isoform 3 (EBI accession no. P15502). The highlighted regions correspond to the signal peptide and hydrophobic and hydrophilic domains. Based on [2]...

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. 

The inner envelope membrane proteins have a cleavable N-terminal transit peptide, as well as some hydrophobic domain (s) in their mature portion. There are two possibilities on the role of this hydrophobic domain it may work as an N-terminal signal peptide after the translocation into the stroma and the subsequent cleavage of the transit peptide. Alternatively, it may work as a stop-transfer signal. One more important question is how the distinction is made between the outer membrane proteins, the inner membrane proteins, and the thylakoid membrane proteins. It is still an enigma. 

Matoba, S., and Ogrydziak, D. (1998). Another factor besides hydrophobicity can affect signal peptide interaction with signal recognition particle. J. Biol. Chem. 273, 18841—... 

Proteins that are destined to be secreted or to end up as transmembrane proteins are synthesized with an N-terminal signal peptide. Signal peptides are highly hydrophobic sequences of variable length. Proteins synthesized with signal peptides are termed preproteins. For example, insulin is a secreted protein. It is therefore synthesized as a preprotein. It is also synthesized in inactive form a proprotein. Insulin as initially synthesized is, in consequence, a preproprotein. 

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