Chitosomes and chitin biogenesis

There appear to be two main types of cytoplasntic vesicles associated with cell wall construction in fungi. The polymers that comprise the amorphous phase of the wall are secreted in macrovesicles (e.g. the conspicuous 'apical' vesicles of mycelial fungi); whereas chitin synthetase, the enzyme needed to make the nticrofibrillar skeleton of the wall of most fungi, is delivered separately in microvesicles (chitosomes). Chitosomes carry chitin synthetase in a zymogenic form. Presumably, zymogenicity delays the onset of chitin synthesis until after the chitosome has reached its destination and the zymogen is activated by surface proteases. Chitin synthetase comprises part of the shell or 'membrane' of the chitosome. It exists as a complex (16S subunit; mol. wt-500 (00) that can be reversibly dissociated with digitonin. The chitin made by dissociated chitosomes has a short, needle-like appearance. Evidently, the spatial organization of enzyme subunits in the chitosome provides conditions conducive to the assembly of long microfibrils.