Signal peptides: exquisitely designed transport promoters

Abstract

Prokaryotic proteins destined for transport out of the cytoplasm typically contain an N-terminal extension sequence, called the signal peptide, which is required for export. It is evident that many secretory proteins utilize a common export system, yet the signal sequences themselves display very little primary sequence homology. In attempting to understand how different signal peptides are able to promote protein secretion through the same pathway, the physical features of natural signal sequences have been extensively examined for similarities that might play a part in function. Experimental data have confirmed statistical analyses which highlighted dominant features of natural signal sequences in Escherichia coli: a net positive charge in the N-terminus increases efficiency of transport; the core region must maintain a threshold level of hydrophobicity within a range of length limitations; the central portion adopts an alpha-helical conformation in hydrophobic environments; and the signal cleavage region is ideally six residues long, with small side-chain amino acids in the -1 and -3 positions. This review focuses on the parallels between signal peptide physical features and their functions, which emerge when the results of a variety of experimental approaches are combined. The requirement for each property may be ascribed to a potential interaction that is critical for efficient protein export. The summation of the key physical features produces signal peptides with the flexibility to function in multiple roles in order to expedite secretion. In this way, nature has indeed evolved exquisitely tuned signal sequences.

Export of a hyperexpressed mammalian globular cytochrome b5 precursor in Escherichia coli is dramatically affected by the nature of the amino acid flanking the secretory signal sequence cleavage bond

“…A ''tighter'' helical conformation induced by bulky, hydrophobic residues such as Ile may have buried the cleavage site within the lipid matrix thus restraining it from gaining access to the exocytoplasmic active site of SPase. These findings are in agreement with previous reports [42][43][44] that overall length of the central core and sequence hydrophobicity determine the in vivo translocation efficiency of export rates. These observations provide the basis for the poorer export rates of the bulky X þ1 and for the peculiar behavior of other less bulky but hydrophilic residues such as Gly.…”

Unusual Characteristics of Amino‐Terminal and Hydrophobic Domains in Nuclear‐Encoded Thylakoid Signal Peptides

“…Numerous theoretical and experimental studies have established the key tenets of bacterial signal peptides (reviewed in Izard and Kendall, 1994), but thylakoidal signal peptides have not been analysed in detail and no mutagenesis studies have been reported. Although only three proteins are known to be transported by the Sec pathway in chloroplasts (PC, 33-kDa PSII protein, and PSI-F), a number of lumenal proteins have been cloned in cyanobacteria, and the sequencing of three algal chloroplast genomes has shown that several lumenal proteins are encoded with signal peptides.…”

“…A large number of studies on prokaryotic Sec signals (reviewed by Izard and Kendall, 1994) have shown quite clearly that the distribution of basic residues is central to the efficiency with which the signal peptide functions. A net positive charge in the N-terminus is important for efficient targeting, with greater charges resulting in more efficient targeting.…”

Characterization of the Mycobacterium tuberculosis erp gene encoding a potential cell surface protein with repetitive structures

“…This ORF ends with a TAA stop codon in position 1270 and potentially encodes a 284 amino acid polypeptide precursor with a calculated molecular mass of 27.6 kDa. The first 22 N-terminal amino acids exhibit structural similarities with signal sequences of exported proteins (Izard & Kendall, 1994;Pugsley, 1993). The putative site of cleavage by signal peptidase is indicated with an arrow in Fig.…”