The presence of a helix breaker in the hydrophobic core of signal sequences of secretory proteins prevents recognition by the signal-recognition particle in Escherichia coli.
Signal sequences often contain alpha-helix-destabilizing amino acids within the hydrophobic core. In the precursor of the Escherichia coli outer-membrane protein PhoE, the glycine residue at position -10 (Gly-10) is thought to be responsible for the break in the alpha-helix. Previously, we showed that substitution of Gly-10 by alpha-helix-promoting residues (Ala, Cys or Leu) reduced the proton-motive force dependency of the translocation of the precursor, but the actual role of the helix breaker remained obscure. Here, we considered the possibility that extension of the alpha-helical structure in the signal sequence resulting from the Gly-10 substitutions affects the targeting pathway of the precursor. Indeed, the mutations resulted in reduced dependency on SecB for targeting in vivo. In vitro cross-linking experiments revealed that the G-10L and G-10C mutant PhoE precursors had a dramatically increased affinity for P48, one of the constituents of the signal-recognition particle (SRP). Furthermore, in vitro cross-linking experiments revealed that the G-10L mutant protein is routed to the SecYEG translocon via the SRP pathway, the targeting pathway that is exploited by integral inner-membrane proteins. Together, these data indicate that the helix breaker in cleavable signal sequences prevents recognition by SRP and is thereby, together with the hydrophobicity of the signal sequence, a determinant of the targeting pathway.
Adams
Hendrik
H
Department of Molecular Microbiology and Institute of Biomembranes, Utrecht University, The Netherlands.
Scotti
Pier A
PA
De Cock
Hans
H
Luirink
Joen
J
Tommassen
Jan
J
D000595:Amino Acid Sequence; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins; D005998:Glycine / Q000737:chemistry; D007700:Kinetics; D008969:Molecular Sequence Data; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D018272:Porins / Q000737:chemistry* / Q000378:metabolism; D014176:Protein Biosynthesis; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D018271:Signal Recognition Particle; D013997:Time Factors; D014158:Transcription, Genetic
D029968:Escherichia coli Proteins; D018272:Porins; D018271:Signal Recognition Particle; C082922:PhoE protein, E coli; D005998:Glycine