Low Km cyclic AMP phosphodiesterase of yeast may be bound to ribosomes associated with the nucleus.

The sub-cellular distribution of low Km cyclic AMP phosphodiesterase (defined as the EDTA-sensitive activity at 1 microM cyclic AMP) was examined using spheroplast lysates and mechanical disintegrates of yeast. Close to 65% of the enzyme was particle-bound in each case. Most of the bound activity in mechanical disintegrates sedimented at 145 000 g in an RNA-rich fraction, and could be solubilised from this fraction by RNase treatment. With spheroplast lysates, however, 50% of the enzyme co-sedimented with DNA at 5 000 g, and the highest specific activity was in purified nuclei with a protein/DNA mass ratio of 16. The results suggest that at least 50% of the enzyme is bound by ribosomes attached to the outer nuclear membrane.