Localization of growth and secretion of proteins in Aspergillus niger

Hyphal growth and secretion of proteins in Aspergillus niger were studied using a new method of culturing the fungus between perforated membranes which allows visualization of both parameters. At the colony level the sites of occurrence of growth and general protein secretion were correlated. In 4-d-old colonies both growth and secretion were localized at the periphery of the colony, whereas in a 5-d-old colony growth and secretion also occurred in a more central zone of the colony where conidiophore differentiation was observed. However, in both cases glucoamylase secretion was mainly detected at the periphery of the colonies. At the hyphal level immunogold labelling showed glucoamylase secretion at the tips of leading hyphae only. Microautoradiography after labelling with N-acetylglucosamine showed that these hyphae were probably all growing. Glucoamylase secretion could not be demonstrated immediately after a temperature shock which stopped growth. These results indicate that glucoamylase secretion is located at the tips of growing hyphae only.