Evidence for the overestimation of molecular masses of proteins after chemical modification and chemical crosslinks on sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE)

Dimerization and actin-bundling properties of villin and its role in the assembly of epithelial cell brush borders

Villin is a major actin-bundling protein in the brush border of epithelial cells. In this study we demonstrate for the first time that villin can bundle actin filaments using a single F-actin binding site, because it has the ability to self-associate. Using fluorescence resonance energy transfer, we demonstrate villin self-association in living cells in microvilli and in growth factor-stimulated cells in membrane ruffles and lamellipodia. Using sucrose density gradient, size-exclusion chromatography, and matrix-assisted laser desorption ionization time-of-flight, the majority of villin was identified as a monomer or dimer. Villin dimers were also identified in Caco-2 cells, which endogenously express villin and Madin-Darby canine kidney cells that ectopically express villin. Using truncation mutants of villin, site-directed mutagenesis, and fluorescence resonance energy transfer, an amino-terminal dimerization site was identified that regulated villin self-association in parallel conformation as well as actin bundling by villin. This detailed analysis describes for the first time microvillus assembly by villin, redefines the actin-bundling function of villin, and provides a molecular mechanism for actin bundling by villin, which could have wider implications for other actin cross-linking proteins that share a villin-like headpiece domain. Our study also provides a molecular basis to separate the morphologically distinct actin-severing and actin-bundling properties of villin.

Arachidonic Acid Allows SNARE Complex Formation in the Presence of Munc18

SNARE complex formation underlies intracellular membrane fusion in eukaryotic organisms; however, the factors regulating the SNARE assembly are not well understood. The neuronal SNARE complex is composed of synaptobrevin2, SNAP-25, and syntaxin1, the latter being under tight control by the cytosolic protein Munc18. We found that the inhibition of syntaxin1 by Munc18 both in nerve terminals and in defined in vitro reactions can be overcome by specific detergents. This serendipitous finding led us to screen biologically relevant fatty acids, revealing that unsaturated arachidonic and linolenic acids can stimulate Munc18-regulated SNARE complex formation in a direct manner. The direct effect of arachidonic acid on the syntaxin1/Munc18 complex suggests a mechanism for the activation of the SNARE assembly pathway and provides a lead for the further investigation of fatty acids that may regulate SNARE-mediated membrane fusion in eukaryotes.

A study of human reovirus IgG and IgA antibodies by ELISA and western blot

Immunoglobulin A and G reactivities to human reovirus were investigated comparatively by Western blot and ELISA in a collection of 120 serum samples. Five different reovirus polypeptides of molecular weights of 127, 79, 54 42 and 40 kDa were visualized by polyacrylamide gel electrophoresis. Of 80 anti-reovirus ELISA-IgG positive serum samples, 97.5% and 15.0% reacted with one or more virus-specific bands in the IgG and IgA Western blot respectively. The 127 and 79 kDa proteins reacted with 100% and 96.2% of the IgG Western blot-positive serum samples (n = 78). IgG reactivities to the 40, 54 and 42 kDa proteins were detected in 96.2%, 80.8% and 20.5% of the immunoblot-positive sera. Compared to the Western blot, the ELISA had a specificity of 94.9% for IgG and 96.3% for IgA detection. The sensitivity of specific IgG and IgA detection by ELISA were 96.3% and 100%, respectively. Using the ELISA, a seroepidemiological survey was carried out on 850 sera collected from October 1989 to September 1992. The overall prevalence of reovirus IgG and IgA antibodies were 53.5% and 15.5% respectively. An age dependent increase of the seroprevalence of anti-reovirus IgG was observed. In contrast, the prevalence of IgA antibodies remained constant during childhood. Seroprevalence peak levels were observed in the 9-11, 20-29, 40-49 and > or = 60 years age group. The results of the study suggest that antireovirus IgG antibodies may persist lifelong, whereas IgA antibodies are detectable for a short time and may indicate a recent reovirus infection. The development of a sensitive and specific ELISA assay for the detection of class-specific immunoglobulins permits further investigation of reovirus infections and may contribute to improving the serological diagnosis of recent infection.