Characterization of the type III secretion locus of Bordetella pertussis

Multiple sequence comparisons of proteins of the LcrD/FlbF family allowed the design of primers that specifically amplify sequences coding for type III secretion components. Amplification of Bordetella pertussis DNA with these primers yielded a fragment that was further used as a probe for screening a genomic library. The nucleotide sequence of a positive clone revealed a 2100-bp gene, called bcrD, which specifies a 75-kDa polypeptide homologous to the Yersinia LcrD protein. Chromosome walking allowed the characterization of a 35-kb DNA segment that contains the entire locus and flanking housekeeping genes. The B. pertussis type III secretion locus consists of more than 30 open reading frames (ORFs), most of which are identical to annotated genes of Bordetella spp and share similarities with known type III secretion genes of related bacteria. In order to assess the function of this locus, we engineered a bcrD null mutant. However, none of the tested phenotypes, such as protein secretion, cellular invasion, cytotoxicity or mouse lung colonization, differentiated the mutant from its parental strain. Studies of bcrD and bscN expressions indicated that, under our experimental conditions, these genes are not expressed in vitro. Restriction analyses on pulsed-field gel electrophoresis allowed the type III locus mapping at coordinate position 1,590 kb on the Tohama I strain chromosome.

Constitutive macropinocytosis in oncogene-transformed fibroblasts depends on sequential permanent activation of phosphoinositide 3-kinase and phospholipase C

Macropinocytosis results from the closure of lamellipodia generated by membrane ruffling, thereby reflecting cortical actin dynamics. Both transformation of Rat-1 fibroblasts by v-Src or K-Ras and stable transfection for expression of dominant-positive, wild-type phosphoinositide 3-kinase (PI3K) regulatory subunit p85 alpha constitutively led to stress fiber disruption, cortical actin recruitment, extensive ruffling, and macropinosome formation, as measured by a selective acceleration of fluid-phase endocytosis. These alterations closely correlated with activation of PI3K and phosphatidylinositol-specific phospholipase C (PI-PLC), as assayed by 3-phosphoinositide synthesis in situ and in vitro and inositol 1, 4,5 trisphosphate steady-state levels, respectively; they were abolished by stable transfection of v-Src-transformed cells for dominant-negative truncated p85 alpha expression and by pharmacological inhibitors of PI3K and PI-PLC, indicating a requirement for both enzymes. Whereas PI3K activation resisted PI-PLC inhibition, PI-PLC activation was abolished by a PI3K inhibitor and dominant-negative transfection, thus placing PI-PLC downstream of PI3K. Together, these data suggest that permanent sequential activation of both PI3K and PI-PLC is necessary for the dramatic reorganization of the actin cytoskeleton in oncogene-transformed fibroblasts, resulting in constitutive ruffling and macropinocytosis.

Intracellular trafficking and membrane translocation of pertussis toxin into host cells

The translocation of the pertussis toxin (PTX) S1 subunit into the cytoplasm of host cells was analysed in CHO cells producing S1 fused to a signal peptide. This protein channelled into the endoplasmic reticulum (ER) by the signal peptide, was found to ADP-ribosylate its target G proteins, suggesting that membrane translocation can occur from the ER and does not require the B oligomer. Similar results were obtained with a C-terminally truncated S1 subunit, indicating that this hydrophobic tail is not involved in the translocation mechanism. We also analysed the activity of two PTX mutants in which the S3 and S2 subunits were substituted for each other. The mutant protein containing two S3 subunits (PTXAS2) presented a decreased binding to fetuin or haptoglobin but higher in vivo activity than the wild-type PTX, suggesting that replacement of S2 by S3 favours the targeting of PTX to the compartment where translocation occurs and/or the dissociation of S1 from the B oligomer, thereby leading to a better translocation of S1 into the cytoplasm.

Regulation of macropinocytosis in v-Src-transformed fibroblasts: cyclic AMP selectively promotes regurgitation of macropinosomes

Stable transformation of Rat-1 fibroblasts by the v-Src oncoprotein results into the constitutive formation of macropinosomes. In the present report, we found that macropinosomes do not fuse with transferrin-containing endosomes and investigated the effects of cyclic AMP as a regulator of macropinocytosis in this cell system. The permeant analogs dibutyryl cyclic AMP and 8-bromo-cyclic AMP, as well as the pharmacological activator of adenylate cyclase forskolin, similarly decreased by about 35% the net endocytic accumulation of the fluid-phase tracer horseradish peroxidase at intervals >5 minutes in v-Src-transformed cells but not in the non-transformed parental Rat-1 cell line. However, and in contrast to the phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N, N-diphenylcarbamate or the phosphatidylinositol 3-kinase inhibitor wortmannin, dibutyryl cyclic AMP neither returned the peroxidase accumulation rate of v-Src-transformed cells to that of parental Rat-1/control cells, nor prevented macropinosome formation, as shown by confocal microscopy. Detailed analysis of the kinetics of tracer entry and efflux in transformed cells revealed that dibutyryl cyclic AMP inhibited peroxidase accumulation only after intervals >5 minutes, due to accelerated peroxidase regurgitation, but did not alter the rate of transferrin recycling. Taken together, these data indicate that, in v-Src-transformed fibroblasts, macropinocytosis and micropinocytosis serve different pathways and that cyclic AMP affects neither micropinocytosis nor the formation of macropinosomes, but selectively promotes regurgitation therefrom.

Three unrelated perturbations similarly uncouple fluid, bulk-membrane, and receptor endosomal flow in rat fetal fibroblasts

We have compared the effects of three perturbations (treatment with 2 microM monensin, potassium depletion, and incubation in 0.35 M NaCl) on recycling of internalized fluid-phase, bulk-membrane, and receptor-mediated tracers in rat fetal fibroblasts. Monensin accelerated 2-fold the regurgitation of the fluid-phase tracer horseradish peroxidase (HRP), as previously described in these cells after potassium depletion or upon incubation in hypertonic medium (1), and all treatments severely inhibited transfer of HRP from endosomes to lysosomes. In comparison, recycling of (6-[N-(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]hexanoyl glucosylsphingosine (C6-NBD-GlcCer), a fluorescent lipid used as a bulk-membrane probe, was not significantly affected while transferrin recycling was slowed down 2-fold. The striking similarities of these unrelated perturbations in their distinct effects on fluid, bulk-membrane, and receptor addressing point to common targets regulating these mechanisms.

v-Src induces constitutive macropinocytosis in rat fibroblasts

The role of v-Src as regulator of fluid-phase pinocytosis was investigated in Rat-1 cells expressing a stable (Rat-1/BB16) or a thermosensitive (Rat-1/tsLA29) v-Src protein. In the second cell line, this protein is inactive when cells are cultured at 40 degrees C but recovers its tyrosine kinase activity upon transfer to 34 degrees C, resulting into a transformed phenotype. The rate of fluid-phase pinocytosis of the tracer horseradish peroxidase was 2-fold higher in v-Src-transformed fibroblasts (Rat-1/BB16, Rat-1/tsLA29 cultured at 34 degrees C) as compared to non-transformed cells (Rat-1, Rat-1/tsLA29 kept at 40 degrees C). In contrast, receptor-mediated endocytosis of transferrin was poorly affected, suggesting that structures distinct from clathrin-coated pits are involved in pinocytosis stimulation. By light and electron microscopy, transformed cells frequently contained large peroxidase-labeled pinocytic vesicles located near to membrane ruffles, demonstrating that stimulation of pinocytosis corresponds to induction of constitutive macropinocytosis. Stimulation of pinocytosis occurred more than 8 hours after transfer to the permissive temperature, whereas transfer to the non-permissive temperature partially reversed the stimulation within 2 hours. Protein synthesis inhibition for 6 hours abrogated pinocytosis stimulation in transformed cells, indicating that constitutive macropinocytosis induced by v-Src depends on continuous synthesis of a short-lived regulatory machinery.

Clathrin polymerization is not required for bulk-phase endocytosis in rat fetal fibroblasts

To assess the role of clathrin in the bulk endocytic flow of rat foetal fibroblasts, the rate of internalization of fluid-phase and membrane-lipid tracers were compared, under control conditions and after inhibition of endocytic clathrin-coated pit formation. After intracellular potassium depletion or upon cell transfer into 0.35 M NaCl, the rate of internalization of receptor-bound transferrin and the residual membrane area of plasmalemmal clathrin-coated pits and vesicles were similarly decreased by approximately 90%. In contrast, the initial rate (< 5 min) of intracellular accumulation of the fluid-phase tracer HRP was not affected. Both in control and treated cells, the rate of HRP accumulation declined after approximately 5 min, and was twofold lower in treated cells, due to enhanced regurgitation. After correction for regurgitation, the endocytic rate constant was similar to measurements at shorter intervals and identical in control and treated cells. Similarly, the rate of internalization and the steady-state level of intracellular accumulation of two fluorescent lipid derivatives, 6-[N-(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]hexanoylglucosylsp hingosine (C6-NBD-GlcCer) and 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH), were not affected by potassium depletion, indicating that the endocytic membrane traffic was equally preserved. Finally, the size distribution of primary endocytic particles that were accessible to HRP within 15 s before glutaraldehyde fixation was also indistinguishable in control and potassium-depleted cells. The simplest explanation is that clathrin polymerization is necessary to concentrate receptor-bound ligands in primary endocytic vesicles, but superfluous to the basic endocytic machinery in rat foetal fibroblasts.

Streptomyces K15 DD-peptidase-catalysed reactions with suicide beta-lactam carbonyl donors

The values of the kinetic parameters that govern the interactions between the Streptomyces K15 DD-peptidase and beta-lactam compounds were determined by measuring the inactivating effect that these compounds exert on the transpeptidase activity of the enzyme and, in the case of [14C]benzylpenicillin and [14C]cefoxitin, by measuring the amounts of acyl-enzyme formed during the reaction. K15 DD-peptidase binds benzylpenicillin or cefoxitin at a molar ratio of 1:1. Benzylpenicilloate is the major product released during breakdown of the acyl-enzyme formed with benzylpenicillin. Benzylpenicillin is not a better acylating agent than the amide Ac2-L-Lys-D-Ala-D-Ala and ester Ac2-L-Lys-D-Ala-D-lactatecarbonyl-donor substrates. beta-Lactam compounds possessing a methoxy group on the alpha-face of the molecule show high inactivating potency.