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Minimum substrate sequence for signal peptidase I of Escherichia coli
1Division of Molecular Genetics and Microbiology, Burroughs Wellcome & Co., Research Triangle Park, North Carolina 27709.
Abstract:
The minimum substrate sequence recognized by signal peptidase I (SPase I or leader peptidase) was defined by measuring the kinetic parameters for a set of chemically synthesized peptides corresponding to the cleavage site of the precursor maltose binding protein (pro-MBP). The minimum sequence of a substrate hydrolyzed by SPase I at a measurable rate was the pentapeptide Ala-Leu-Ala decreases Lys-Ile. The rates of hydrolysis of this substrate, however, were several hundred-fold lower than those observed for the maturation of MBP in Escherichia coli, suggesting that in addition to these minimal sites involved in recognition, other features of pro-MBP are also needed for the optimal rate of signal peptide cleavage by SPase I. One parameter may be the length of the polypeptide chain. Studies of the synthetic peptides showed that decreasing the length of the polypeptide chain of substrates decreased the substrate efficiency measured as kcat/Km. However, in one case a decrease in the length of a peptide corresponding to -7 to +3 positions of pro-MBP to a nonapeptide (-7 to +2) increased the substrate efficiency by about 900-fold. The nonapeptide is the most efficient substrate for the enzyme in vitro so far reported. It is speculated that better peptide substrates are the ones which are able to adopt folded structures.
Insights
Researchers identified the minimal substrate sequence for signal peptidase I (SPase I), a crucial enzyme in protein maturation. Optimal cleavage efficiency, however, depends on additional factors beyond this minimal recognition site.
Area of Science:
- Biochemistry
- Molecular Biology
- Enzymology
Background:
- Signal peptidase I (SPase I) is essential for processing precursor proteins, including maltose binding protein (MBP).
- Understanding the substrate specificity of SPase I is critical for elucidating protein maturation pathways in prokaryotes like Escherichia coli.
Purpose of the Study:
- To define the minimum substrate sequence recognized by SPase I.
- To investigate the influence of substrate length and structure on SPase I cleavage efficiency.
Main Methods:
- Chemical synthesis of peptide substrates corresponding to the pro-MBP cleavage site.
- Kinetic parameter measurements (kcat/Km) to assess substrate hydrolysis rates.
- Comparative analysis of different peptide lengths and their impact on enzyme efficiency.
Main Results:
- The minimal recognition sequence for SPase I was determined to be a pentapeptide (Ala-Leu-Ala-Ala-Lys-Ile).
- Hydrolysis rates for this minimal sequence were significantly lower than in vivo maturation rates.
- A nonapeptide (-7 to +2) derived from pro-MBP exhibited a 900-fold increase in substrate efficiency in vitro.
- Substrate efficiency generally decreased with shorter polypeptide chains, with notable exceptions.
Conclusions:
- SPase I recognition involves a minimal sequence, but optimal cleavage requires additional features of the precursor protein.
- Polypeptide chain length and potential for folded structures significantly influence SPase I substrate efficiency.
- The nonapeptide (-7 to +2) represents the most efficient in vitro substrate reported for SPase I to date.