Threshold level of protein kinase A activity and polarized growth in Mucorrouxii

A model system to study the involvement of cAMP-mediated regulation of a ce llular process such as hyphal morphogenesis was investigated. Impairment of polarized growth was observed when Mucor rouxii sporangiospores were grown in the presence of N-6-cAMP analogues and of SQ 65,442, a cAMP phosphodies terase inhibitor. Together with an effect on isodiametric growth, there was increased pigmentation, increased cell fragility and loss of cell adhesive ness. The total effect on morphology was attained even after adding the com pounds shortly before germ-tube emergence; when added after this time growt h continued in a non-polarized form and rounding of the germ tip was observ ed. The morphological effect was observed under all the nutritional and env ironmental conditions studied (aerobic conditions and defined medium with m altose or glucose, Casamino acids medium with glucose, or rich medium; anae robic conditions with rich medium; and following a shift from anaerobiosis to aerobiosis). The time of germ-tube emergence, and the size of the cell a t this time, was dependent on the growth medium. Protein kinase A (PKA) spe cific activity was followed during the germination process under three grow th conditions. It was found that the total activity of PKA correlated with differentiation and not with growth, and that the total specific activity a t the time of germination was the same, independent of the culture medium. The time of germ-tube emergence correlated with the time of attainment of a threshold level of PKA total specific activity. The concentration of dibut yryl-cAMP needed to promote isodiametric growth correlated with the total u nits of activity of PKA to be activated per cell. It was concluded that PKA is involved in the morphogenetic process of the fungus grown under all the nutritional and ambient conditions tested.