A chemically synthesized radiolabeled signal peptide: design, preparation, and biological evaluation of an iodinated analog of preproparathyroid hormone

Type I signal peptidases of Gram-positive bacteria

Proteins that are exported from the cytoplasm to the periplasm and outer membrane of Gram-negative bacteria, or the cell wall and growth medium of Gram-positive bacteria, are generally synthesized as precursors with a cleavable signal peptide. During or shortly after pre-protein translocation across the cytoplasmic membrane, the signal peptide is removed by signal peptidases. Importantly, pre-protein processing by signal peptidases is essential for bacterial growth and viability. This review is focused on the signal peptidases of Gram-positive bacteria, Bacillus and Streptomyces species in particular. Evolutionary concepts, current knowledge of the catalytic mechanism, substrate specificity requirements and structural aspects are addressed. As major insights in signal peptidase function and structure have been obtained from studies on the signal peptidase LepB of Escherichia coli, similarities and differences between this enzyme and known Gram-positive signal peptidases are highlighted. Notably, while the incentive for previous research on Gram-positive signal peptidases was largely based on their role in the biotechnologically important process of protein secretion, present-day interest in these essential enzymes is primarily derived from the idea that they may serve as targets for novel anti-microbials.

The structure and mechanism of bacterial type I signal peptidases. A novel antibiotic target

Type I signal peptidases are essential membrane-bound serine proteases that function to cleave the amino-terminal signal peptide extension from proteins that are translocated across biological membranes. The bacterial signal peptidases are unique serine proteases that utilize a Ser/Lys catalytic dyad mechanism in place of the classical Ser/His/Asp catalytic triad mechanism. They represent a potential novel antibiotic target at the bacterial membrane surface. This review will discuss the bacterial signal peptidases that have been characterized to date, as well as putative signal peptidase sequences that have been recognized via bacterial genome sequencing. We review the investigations into the mechanism of Escherichia coli and Bacillus subtilis signal peptidase, and discuss the results in light of the recent crystal structure of the E. coli signal peptidase in complex with a beta-lactam-type inhibitor. The proposed conserved structural features of Type I signal peptidases give additional insight into the mechanism of this unique enzyme.

Correlation of secondary structure with biological activity for a leader peptide: circular dichroism-derived structure and in vitro biological activities of preproparathyroid hormone peptide and its analogs

Leader or signal sequences are specialized domains within precursor proteins which serve an essential role in interacting with the cellular secretory apparatus to enable intracellular transport and secretion of proteins. Despite many differences in primary amino acid sequences, signal domains interact with a common set of intracellular components, presumably because the signal sequences share an overall conformational similarity. In a few instances, mutant signal peptides from prokaryotes have been studied and their structures correlated with function (export) in vivo. A series of analogs of the precursor-specific region of preproparathyroid hormone have been prepared which contain substitutions of either proline or a charged amino acid within the hydrophobic core. These synthetic "mutants" have previously been evaluated in several in vitro assays to determine their functionality with regard to protein secretion and suitability as substrates for signal peptidase. The secondary structural content of each peptide, as well as the native sequence and sulfur-free analog, was determined in aqueous and nonaqueous conditions by circular dichroism (CD) as a function of time. The structures obtained were correlated with in vitro bioactivities. Unlike the findings or previous CD studies, all the peptides examined here had low to undetectable alpha-helical content in both aqueous and nonaqueous buffers. The unsubstituted and sulfur-free analogs had high (80-85%) beta-structure in aqueous conditions which was reduced to approximately 30% in nonaqueous solvent. The proline- and charged-substituted peptides contained about half the beta-structure content (35-55%) in aqueous buffer; in nonaqueous solvent their structure was similar to the unsubstituted peptides. The structure-activity correlates found were as follows: a high degree of structure (aqueous conditions) correlated with interaction with signal recognition particle and substrate suitability for signal peptidase; a low degree of structure (nonaqueous environment) correlated with activity in the translocation assay.