PROTEIN PHOSPHATASE 2A IS INVOLVED IN HYPHAL GROWTH OF NEUROSPORA-CRASSA

Cantharidin and calyculin A, natural toxins that are inhibitors of pro tein phosphatases 1 and 2A (PP1 and PP2A, respectively), inhibit Neuro spora crassa hyphal growth. When N. cassa was grown in the presence of either drug, abnormalities were observed at hyphal tips. In addition, both drugs induced an increase in hyphal branching. Cantharidin inhib ited N. crassa hyphal growth in a temperature-dependent manner, as the effect of the drug was more pronounced at 34 degrees C than at 25 deg rees C. In addition to the drug-mediated inhibition of phosphatase act ivity, a genetic approach was used to determine the phenotypic consequ ences of reduced PP2A activity. Two strains with subnormal PP2A activi ty were constructed. The first, in which the original pph-1 gene (enco ding the PP2A catalytic subunit) was replaced with an ectopically inte grated copy of pph-1, exhibited lower levels of pph-1 transcript, lowe r PP2A activity and increased sensitivity to cantharidin. Similarly, i n a second strain, in which the pph-1 gene was cloned in an antisense orientation downstream of the inducible isocitrate lyase promoter, low er levels of pph-1 transcript, as well as of PP2A activity, and a redu ction in hyphal growth were observed. The results of this study indica te that PP2A, and probably other Ser/Thr phosphatases, are involved in the regulation of hyphal growth in N. crassa.