Mapping the binding site of colchicinoids on beta -tubulin. 2-Chloroacetyl-2-demethylthiocolchicine covalently reacts predominantly with cysteine 239 and secondarily with cysteine 354

2-Chloroacetyl-2-demethylthiocolchicine (2CTC) and 3-chloroacetyl-3-demethylthiocolchicine (3CTC) resemble colchicine in binding to tubulin and react covalently with beta-tubulin, forming adducts with cysteine residues 239 and 354. The adducts at Cys-239 are less stable than those at Cys-354 during formic acid digestion. Extrapolating to zero time, the Cys-239 to Cys-354 adduct ratio is 77:23 for 2CTC and 27:73 for 3CTC. Using energy minimization modeling to dock colchicinoids into the electron crystallographic model of beta-tubulin in protofilaments (Nogales, E. , Wolf, S. G., and Downing, K. H. (1998) Nature 391, 199-203), we found two potential binding sites. At one, entirely encompassed within beta-tubulin, the C2- and C3-oxygen atoms of 2CTC and 3CTC overlapped poorly with those of colchicine and thiocolchicine, but distances from the reactive carbon atoms of the analogs to the sulfur atoms of the cysteine residues were qualitatively consistent with reactivity. The other potential binding site was located at the alpha/beta interface. Here, the oxygen atoms of the analogs overlapped well with those of colchicine, but relative distances of the reactive carbons to the cysteine sulfur atoms did not correlate with the observed reactivity. A significant conformational change must occur in the colchicine binding site of tubulin in the transition from the unpolymerized to the polymerized state.

Direct photoaffinity labeling of cysteine 211 or a nearby amino acid residue of beta-tubulin by guanosine 5'-diphosphate bound in the exchangeable site

Tubulin with [8-14C]GDP bound in the exchangeable site was exposed to ultraviolet light, and radiolabel was cross-linked to two peptide regions of the beta-subunit. Following enrichment for peptides cross-linked to guanosine by boronate chromatography, we confirmed that the cysteine 12 residue was the major site of cross-linking. However, significant radiolabel was also incorporated into a peptide containing amino acid residues 206 through 224. Although every amino acid in this peptide except cysteine 211 was identified by sequential Edman degradation, implying that this was the amino acid residue cross-linked to guanosine, radiolabel at C-8 was usually lost during peptide processing (probably during chromatography at pH 10). Consequently, the radiolabeled amino acid could not be unambiguously identified.

Mutant analysis of Prevotella sp. plaA-lacZ fusion protein expression in Escherichia coli: support for an essential role of the stem-loop

This study investigated the involvement of RNA folding in the synthesis of a fusion protein with beta-galactosidase activity. The coding gap region of the Prevotella loescheii adhesin gene plaA was fused in-frame with the Escherichia coli lacZ gene on plasmid pSK105. N-Terminal sequencing of the expressed plaA-lacZ protein indicated that it resulted from translational initiation at a fortuitous ribosomal-binding site within the plaA sequence at nt 570. Specific mutations were introduced in the stem-loop region that precedes the gap sequence. Analysis of stem-loop mutants, together with the introduction of compensatory mutations that restored activity, supports a requirement for stem-loop formation within the plaA sequence preceding the translational initiation site. A mutation reducing the predicted size of the loop, but preserving the stem structure, inactivated fusion protein synthesis. A suppressor mutation predicted to restore the size of the loop restored efficient fusion protein synthesis. In addition, the sequence preceding the translational start site of the plaA-lacZ fusion has several similarities to sequences that function as translational enhancers in prokaryotes. These include a stem-loop structure, an A-U rich region preceding the initiation codon, and a region of homology to 16S rRNA.

Direct photoaffinity labeling of cysteine-295 of alpha-tubulin by guanosine 5'-triphosphate bound in the nonexchangeable site

The alphabeta-tubulin heterodimer has two high affinity guanosine 5'-triphosphate binding sites, so that purified tubulin usually contains two molecules of bound guanosine nucleotide. Half this nucleotide is freely exchangeable with exogenous guanine nucleotide, and its binding site has been readily localized to the beta-subunit. The remaining nonexchangeable guanosine 5'-triphosphate can only be released from tubulin by denaturing the protein. We replaced the exchangeable site nucleotide of tubulin with 2'-deoxyguanosine 5'-diphosphate, exposed the resulting tubulin to ultraviolet light, degraded the protein, and isolated ribose-containing peptide derived from the nonexchangeable site. A large cyanogen bromide peptide was recovered, and its further degradation with endoproteinase Glu-C established that cysteine-295 of alpha-tubulin was the major reactive amino acid cross-linked to guanosine by ultraviolet irradiation.

In vitro inhibition of murine macrophage migration by Bordetella pertussis lymphocytosis-promoting factor

Lymphocytosis promoting factor (LPF) of Bordetella pertussis is a protein toxin which may have a role in the pathogenesis of pertussis. Since macrophages have an important role in the control of respiratory infections, the in vitro effects of LPF on macrophages from C3H/HeN and C3H/HeJ mice and on a murine macrophage-like cell line, RAW264, were examined. LPF inhibited random migration of resident peritoneal macrophages as well as the chemotaxis of peritoneal macrophages and the cell line. Fifty percent inhibition of chemotaxis occurred at 0.2 to 0.3 ng of LPF per ml for the macrophages and at 1 to 2 ng of LPF per ml for the cell line. When LPF was either heated at 80 degrees C for 5 min or premixed with specific antibodies, it failed to inhibit migration. At 20 ng/ml, LPF inhibited chemotaxis by more than 80% and also decreased Fc-mediated phagocytosis by 25 to 35%. At this dose, LPF was not a chemoattractant for murine macrophages and did not reduce macrophage viability, adherence, or opsonized zymosan-stimulated superoxide release. When LPF-treated macrophages were added to tissue culture dishes and then examined microscopically after 4 h, the LPF-treated cells adhered but failed to spread and elongate as well as control macrophages. These data indicate that LPF specifically inhibits macrophage migration in vitro and suggest that a possible role for LPF in pathogenesis is to inhibit migration of macrophages to the site of B. pertussis infection.