Substrate specificity of a peptidyl-aminoacyl-L/D-isomerase from frog skin.

In the skin of fire-bellied toads (Bombina species), an aminoacyl-L/D-isomerase activity is present which catalyses the post-translational isomerization of the L- to the D-form of the second residue of its substrate peptides. Previously, this new type of enzyme was studied in some detail and genes potentially coding for similar polypeptides were found to exist in several vertebrate species including man. Here, we present our studies to the substrate specificity of this isomerase using fluorescence-labeled variants of the natural substrate bombinin H with different amino acids at positions 1, 2 or 3. Surprisingly, this enzyme has a rather low selectivity for residues at position 2 where the change of chirality at the alpha-carbon takes place. In contrast, a hydrophobic amino acid at position 1 and a small one at position 3 of the substrate are essential. Interestingly, some peptides containing a Phe at position 3 also were substrates. Furthermore, we investigated the role of the amino-terminus for substrate recognition. In view of the rather broad specificity of the frog isomerase, we made a databank search for potential substrates of such an enzyme. Indeed, numerous peptides of amphibia and mammals were found which fulfill the requirements determined in this study. Expression of isomerases with similar characteristics in other species can therefore be expected to catalyze the formation of peptides containing D-amino acids.

Single amino acid repeats in signal peptides.

There has been an increasing interest in single amino acid repeats ever since it was shown that these are the cause of a variety of diseases. Although a systematic study of single amino acid repeats is challenging, they have subsequently been implicated in a number of functional roles. In general surveys, leucine runs were among the most frequent. In the present study, we present a detailed investigation of repeats in signal peptides of secreted and type I membrane proteins in comparison with their mature parts. We focus on eukaryotic species because single amino acid repeats are generally rather rare in archaea and bacteria. Our analysis of over 100 species shows that repeats of leucine (but not of other hydrophobic amino acids) are over-represented in signal peptides. This trend is most pronounced in higher eukaryotes, particularly in mammals. In the human proteome, although less than one-fifth of all proteins have a signal peptide, approximately two-thirds of all leucine repeats are located in these transient regions. Signal peptides are cleaved early from the growing polypeptide chain and then degraded rapidly. This may explain why leucine repeats, which can be toxic, are tolerated at such high frequencies. The substantial fraction of proteins affected by the strong enrichment of repeats in these transient segments highlights the bias that they can introduce for systematic analyses of protein sequences. In contrast to a general lack of conservation of single amino acid repeats, leucine repeats were found to be more conserved than the remaining signal peptide regions, indicating that they may have an as yet unknown functional role.

Bombinins, antimicrobial peptides from Bombina species.

The skin secretions of Bombina species contain peptides and small proteins with interesting biological properties. These include bombesin, thyrotropin releasing hormone, BSTI and Bv8. In this review, the biosynthesis and antimicrobial activity of two groups of peptides, bombinins and bombinins H, are described. To date, these have only been found in Bombina skin. They are derived from common precursors containing one or two bombinin copies at the amino and a single bombinin H at the carboxyl end. Bombinins are active against Gram-positive and Gram-negative bacteria and fungi but virtually inactive in haemolysis assays. Conversely, bombinins H have lower bactericidal activities but lyse erythrocytes. In the skin secretions, bombinins H are present in two sizes with either 20 or 17 amino acids. Moreover, they occur as epimers with either an L- or a D-amino acid at position 2. An enzyme catalyzing this inversion of chirality of an amino acid in peptide linkage has been isolated from Bombina skin secretions. In different tests, also with different stages of the life cycle of Leishmania parasites, the d-forms were found to be more active. Biophysical studies have yielded some insight into the different behaviours of the epimers in model membranes.

Biosynthesis of a D-amino acid in peptide linkage by an enzyme from frog skin secretions.

d-amino acids are present in some peptides from amphibian skin. These residues are derived from the corresponding L-amino acids present in the respective precursors. From skin secretions of Bombinae, we have isolated an enzyme that catalyzes the isomerization of an L-Ile in position 2 of a model peptide to D-allo-Ile. In the course of this reaction, which proceeds without the addition of a cofactor, radioactivity from tritiated water is incorporated into the second position of the product. The amino acid sequence of this isomerase could be deduced from cloned cDNA and genomic DNA. After expression of this cDNA in oocytes of Xenopus laevis, isomerase activity could be detected. Polypeptides related to the frog skin enzyme are present in several vertebrate species, including humans.

The AVIT protein family. Secreted cysteine-rich vertebrate proteins with diverse functions.

Homologues of a protein originally isolated from snake venom and frog skin secretions are present in many vertebrate species. They contain 80-90 amino acids, 10 of which are cysteines with identical spacing. Various names have been given to these proteins, such as mamba intestinal protein 1 (MIT1), Bv8 (Bombina variegata molecular mass approximately 8 kDa), prokineticins and endocrine-gland vascular endothelial growth factor (EG-VEGF). Their amino-terminal sequences are identical, and so we propose that the sequence of their first four residues, AVIT, is used as a name for this family. From a comparison of the sequences, two types of AVIT proteins can be discerned. These proteins seem to be distributed widely in mammalian tissues and are known to bind to G-protein-coupled receptors. Members of this family have been shown to stimulate contraction of the guinea pig ileum, to cause hyperalgesia after injection into rats and to be active as specific growth factors. Moreover, the messenger RNA level of one of these AVIT proteins changes rhythmically in the region of the brain known as the suprachiasmatic nucleus. This shows that members of this new family of small proteins are involved in diverse biological processes.

'Piggy-back' transport of Xenopus hyaluronan synthase (XHAS1) via the secretory pathway to the plasma membrane.

Hyaluronan is the sole glycosaminoglycan whose biosynthesis takes place directly at the plasma membrane. The mechanism by which hyaluronan synthase (HAS) becomes inserted there, as well as the question of how the enzyme discriminates between particular membrane species in polarized cells, are largely unknown. In vitro translation of HAS suggested that the nascent protein becomes stabilized in the presence of microsomal membranes, but would not insert spontaneously into membranes after being translated in the absence of those. We therefore monitored the membrane attachment of enzymatically active fusion proteins consisting of Xenopus HAS1 and green fluorescent protein shortly after de novo synthesis in Vero cells. Our data strongly suggest that HAS proteins are directly translated on the ER membrane without exhibiting an N-terminal signal sequence. From there the inactive protein is transferred to the plasma membrane via the secretory pathway. For unknown reasons, HAS inserted into membranes other than the plasma membrane remains inactive.

Nociceptive sensitization by the secretory protein Bv8.

1 The small protein Bv8, isolated from amphibian skin, belongs to a novel family of secretory proteins (Bv8-Prokineticin family, SWISS-PROT: Q9PW66) whose orthologues have been conserved throughout evolution, from invertebrates to humans. 2 When injected intravenously or subcutaneously (from 0.06 to 500 pmol kg(-1)) or intrathecally (from 6 fmol to 250 pmol) in rats, Bv8 produced an intense systemic nociceptive sensitization to mechanical and thermal stimuli applied to the tail and paws. 3 Topically delivered into one rat paw, 50 fmol of Bv8 decreased by 50% the nociceptive threshold to pressure in the injected paw without affecting the threshold in the contralateral paw. 4 The two G-protein coupled prokineticin receptors, PK-R1 and PK-R2, were expressed in rat dorsal root ganglia (DRG) and in dorsal quadrants of spinal cord (DSC) and bound Bv8 and the mammalian orthologue, EG-VEGF, with high affinity. In DSC, PK-R1 was more abundant than PK-R2, whereas both receptors were equally expressed in DRG. IC(50) of Bv8 and EG-VEGF to inhibit [(125)I]-Bv8 binding to rat DRG and DSC were 4.1+/-0.4 nM Bv8 and 76.4+/-7.6 nM EG-VEGF, in DRG; 7.3+/-0.9 nM Bv8 and 330+/-41 nM EG-VEGF, in DSC. 5 In the small diameter neurons (<30 microm) of rat DRG cultures, Bv8 concentrations, ranging from 0.2 to 10 nM, raised [Ca(2+)](i) in a dose-dependent manner. 6 These data suggest that Bv8, through binding to PK receptors of DSC and primary sensitive neurons, results in intense sensitization of peripheral nociceptors to thermal and mechanical stimuli.