Quantitative in vitro biopolymerization to chitin in native chitosomal membranes supported by silica microparticles

To investigate the unknown physical mechanisms of chitin biosynthesis quantitatively, we designed a quantitative in vitro biopolymerization assay by deposition of native chitosomal membranes from Saccharomyces cerevisiae onto solid silica microparticles of a defined size (ø = 3 microm). The homogeneous coating of particle surfaces with native chitosomal membranes observed by confocal microscopy agrees well with the surface coverage calculated by the phosphate analysis. The amount of the synthesized chitin polymers is determined by radioactive assays, which demonstrate that chitin synthase in particle-supported membranes retains its specific enzymatic activity. In comparison to planar substrates, particle supports of defined size (and thus surface area) enable us to amplify the signals from immobilized proteins owing to the much larger surface area and to the capability of concentrating the sample to any given sample volume. Moreover, the large density of particle supports offers unique advantages over purified chitosomes in the quick separation of particle-supported membranes and materials in bulk within 1 min. This allows for the termination of the polymerization reaction without the disruption of the whole membranes, and thus the chitin polymers released in bulk can quantitatively be extracted. The obtained results demonstrate that the native biological membranes on particle supports can be utilized as a new in vitro biopolymerization assay to study the function of transmembrane enzyme complexes.