Synthesis of alpha-factor analogues containing photoactivatable and labeling groups

Position 13 analogs of the tridecapeptide mating pheromone from Saccharomyces cerevisiae: design of an iodinatable ligand for receptor binding

Analogs of the alpha-factor tridecapeptide mating pheromone (WHWLQLKPGQPMY) from Saccharomyces cerevisiae in which Tyr13 was replaced with Phe, p-F-Phe, m-F-Phe, p-NO2-Phe, p-NH2-Phe or Ser were synthesized and purified to >99% homogeneity. These analogs were bioassayed using a growth arrest assay and a gene induction assay and evaluated for their ability to compete with binding of tritiated alpha-factor to its receptor Ste2p. The results showed that the phenolic OH of Tyr13 is not required for either biological activity or receptor recognition. Analogs containing fluorine, amino, nitro or a hydrogen in place of OH had 80-120% of the biological activity of the parent pheromone in the gene induction assay and had receptor affinities from nearly equal to 6-fold lower than that of alpha-factor. In contrast, substitution of Ser or Ala at position 13 resulted in a >100-fold decrease in receptor affinity suggesting that the aromatic ring is involved in binding to the receptor. The lack of a strict requirement for Tyr13 allowed the design of several multiple replacement analogs in which Phe or p-F-Phe were substituted at position 13 and Tyr was placed in other positions of the peptide. These analogs could then be iodinated and used in the development of a highly sensitive receptor-binding assay. One potential receptor ligand [Tyr(125I)1,Nle12, Phe13] alpha-factor exhibited saturable binding with a KD of 81 nM and was competed by alpha-factor for binding in a whole-cell assay. Thus a new family of radioactive ligands for the alpha-factor receptor has been revealed. These ligands should be extremely useful in defining active site residues during mutagenesis and cross-linking studies.

Position one analogs of the Saccharomyces cerevisiae tridecapeptide pheromone

Analogs of the Saccharomyces cerevisiae alpha-mating factor [WHWLQLKPGQPMY], in which a variety of residues replaced Trp1 were synthesized and assayed for biological activity and receptor affinity. Analogs containing Gly or Leu or many different aromatic residues in position 1 of the peptide exhibited bioactivity in a growth arrest assay slightly greater than, or equal to, that of the parent pheromone, whereas the Glu1 and Lys1 analogs exhibited significantly lower bioactivity. Analogs with an aromatic replacement at position 1 had 3- to 6-fold lower receptor affinity than the parent peptide, whereas analogs with a hydrophilic residue at the N-terminus exhibited large reductions in receptor affinity with the peptide with Glu in position 1 showing a 120-fold reduction. N alpha-Acetylation had little effect on bioactivity but lowered receptor affinity by 20- to 40-fold. Amidation of the carboxyl terminus resulted in a 10-fold decrease in activity and a 160-fold decrease in receptor affinity. These results indicate that the alpha-factor receptor has a large hydrophobic binding pocket, possibly containing a negatively charged side-chain, which interacts with the N-terminus of alpha-factor. The lack of correlation between activity and binding and several analogs suggests that small residues near the N-terminus of alpha-factor may be very efficient in triggering isomerization of the receptor to its activated state in the first step of the signal transduction pathway.

Benzophenone photoprobes for phosphoinositides, peptides and drugs

Benzophenones (BP) and related aryl ketone photophores have become established as the photoactivatable group of choice for high-efficiency covalent modification of hydrophobic regions of binding proteins, including enzymes and receptors that recognize peptide hormones, (oligo) nucleotides and nucleosides, phosphoinositides, inositol polyphosphates and a wide variety of therapeutic molecules. This review presents the advantages of BP as photoaffinity labels and provides specific examples from the last 3 years of applications of BP-containing ligands used in biochemistry.

Photochemical conjugation of peptides to carrier proteins using 1,2,3-thiadiazole-4-carboxylic acid. Immunoreactivity of free C-terminal epitope with specific antibodies

A new heterobifunctional cross-linking reagent, 1,2,3-thiadiazole-4-carboxylic acid, for the photochemical conjugation of peptides to proteins is described. The title compound can be coupled directly to a protected peptide resin during solid-phase peptide synthesis (SPPS) using standard coupling procedures. The probe is stable to TFA deprotection/cleavage mixtures containing ethanedithiol commonly used in Fmoc-SPPS. Furthermore, tritium may easily be introduced into the thiadiazole ring by base-catalyzed hydrogen-exchange. Upon irradiation at 245-300 nm, parent 1,2,3-thiadiazole rapidly eliminates N2, generating very reactive thioketene which reacts with amines to give a thioamide in high yield, even when the photolysis is carried out in hydroxylic solvents. In order to investigate the potential of the title compound as a heterobifunctional cross-linking reagent a model study with angiotensin II (AII) was conducted. The photoreactive peptide N2-4-carbonyl-1,2,3-thiadiazole-AII (TDA-AII) was synthesized by Fmoc-SPPS and conjugated to bovine serum albumin (BSA) by photolysis at 245 and 300 nm. By use of a capture competition ELISA, the C-terminal Pro-Phe epitope of photoconjugated AII with the sequence DRVYIHPF was shown to bind specifically to antiAII antibodies (anti-AII abs), although antibodies against both the C- and N-terminal epitopes were present in the assay. A dipeptide His-Leu carboxy-extension form of AII, angiotensin I (AI), only bound to anti-AII abs at 100-200 times higher concentrations, showing that the C-terminal epitope was blocked by the dipeptide.