Regulation of protein kinase A subunits during germination of Mucor rouxii sporangiospores

Levels of protein kinase A (PKA) subunits and of cAMP have been measured during aerobic germination of the sporangiospores of the dimorphic fungus Mucor rouxii ; further, the holoenzyme and its catalytic (C) and regulatory (R) subunits have been visualized through sucrose gradient centrifugation. Sporangiospores contain around 0.06 μM of a dimeric holoenzyme species of 5.5 S and a sixfold excess of a free R subunit of 2.7 S. Both these species are proposed to be derived by proteolysis from the native forms. Enzymic activity at this stage is highly inhibited, as demonstrated with permeabilized cells. Immediately upon germination, and after a transient increase in cAMP concentration from 10 μM to 90 μM, C‐subunit levels fall to 30%. After the onset of germination, the specific activity and concentration of both the 5.5 S holoenzyme species and the 2.7 S species of free R subunit decrease in parallel to the increase in total protein and volume. Net synthesis of C and R subunits to form a native holoenzyme species of 8.8 S is apparent 4 h onwards after germination. A significant increase in cellular concentration is observed at 6 h. At 7 h of growth, when germ‐tube emission is complete, the holoenzyme concentration is around 0.23 μM; there is almost no free R subunit and the intracellular concentration of cAMP is around 3 μM. A role for PKA during germination and morphogenesis is discussed.