APOLAR GROWTH OF NEUROSPORA-CRASSA LEADS TO INCREASED SECRETION OF EXTRACELLULAR PROTEINS

Protein secretion in filamentous fungi has been shown to be restricted to actively growing hyphal tips. To determine whether an increase in the amount of growing surface area of a fungus can lead to an increase in the amount of protein secretion, we examined secretion in a temper ature-sensitive Neurospora crassa mcb mutant that shows a loss of grow th polarity when incubated at restrictive temperature. Incubation of t he mob mutant at restrictive temperature results in a three- to fivefo ld increase in the level of extracellular protein and a 20-fold increa se in carboxymethyl cellulase activity relative to a wild-type strain. A mutation in the cr-l gene has been shown previously to suppress the apolar growth phenotype of the mob mutant, and we find that the level of extracellular protein produced by a mcb;cr-1 double mutant was red uced to that of the wild-type control. Immunolocalization of a secrete d endoglucanase revealed that proteins are secreted mainly at hyphal t ips in hyphae exhibiting polar growth and over the entire surface area of bulbous regions of hyphae that are produced after a shift of the m cb mutant to restrictive temperature. These results support the hypoth esis that secretion of extracellular protein by a filamentous fungus c an be significantly increased by mutations that alter growth polarity.