Discovery of functional motifs in h-regions of trypanosome signal sequences.
N-terminal signal peptides direct secretory proteins into the ER (endoplasmic reticulum) of eukaryotes or the periplasmic space of prokaryotes. A hydrophobic core (h-region) is important for signal sequence function; however, the mechanism of h-region action is not resolved. To gain new insight into signal sequences, bioinformatic analysis of h-regions from humans, Saccharomyces cerevisiae, Trypanosoma brucei and Escherichia coli was performed. Each species contains a unique set of peptide motifs (h-motifs) characterized by identity components (i.e. sequence of conserved amino acids) joined by spacers. Human h-motifs have four identity components, whereas those from the other species utilize three identity components. Example of h-motifs are human Hs3 {L-x(2)-[AGILPV]-L-x(0,2)-L}, S. cerevisiae Sc1 [L-x(0,2)-S-x(0,3)-A], T. brucei Tb2 {L-x(1,2)-L-[AILV]} and E. coli Ec1 [A-x(0,2)-L-x(0,3)-A]. The physiological relevance of h-motifs was tested with a T. brucei microsomal system for translocation of a VSG (variant surface glycoprotein)-117 signal peptide. Disruption of h-motifs by scrambling of sequences in h-regions produced defective signal peptides, although the hydrophobicity of the peptide was not altered. We conclude that: (i) h-regions harbour h-motifs, and are not random hydrophobic amino acids; (ii) h-regions from different species contain unique sets of h-motifs; and (iii) h-motifs contribute to the biological activity of ER signal peptides. h-Regions are 'scaffolds' in which functional h-motifs are embedded. A hypothetical model for h-motif interactions with a Sec61p protein translocon is presented.
Duffy
Josh
J
Department of Cellular Biology, 724 Biological Sciences Building, University of Georgia, Athens, GA 30602, USA.
Patham
Bhargavi
B
Mensa-Wilmot
Kojo
K
D000595:Amino Acid Sequence; D000818:Animals; D004721:Endoplasmic Reticulum / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D056890:Eukaryota / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D011485:Protein Binding; D021382:Protein Sorting Signals; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D015800:Protozoan Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D014346:Trypanosoma brucei brucei / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
D029968:Escherichia coli Proteins; D021382:Protein Sorting Signals; D015800:Protozoan Proteins