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A plausible role for a membrane-bound cyclic AMP phosphodiesterase in cellular slime mold chemotaxis
Abstract:
1. Kinetics of membrane-bound cyclic AMP phosphodiesterase of the cellular slime mold, Dictyostelium discoideum, were studied under two conditions: in the 27 000 times g sediment of cell homogenates (particle-bound phosphodiesterase) and in cell suspensions using external cyclic AMP as a substrate (cell-bound phosphodiesterase). Both methods revealed non-Michaelian kinetics with interaction coefficients less than 1. 2. The membrane-bound phosphodiesterase has a specificity different from that of the cyclic AMP receptor, also present at the cell surface. 3. The membrane-bound enzyme was solubilized by lithium 3, 5-diiodosalicylate and partially purified. In this state the non-linear kinetics were still retained; however, the enzyme was not inhibited by the D. discoideum inhibitor, unlike the cell-bound phosphodiesterase in vivo. This indicates that both enzymes share an inhibitor binding site and that this site is cryptic in the cell-bound state. 4. Production of periodic cyclic AMP pulses by centers, and their relay by other cells, is believed to occur during aggregation. It is suggested that the cell-bound enzyme determines a "time window" significantly smaller than the period of pulsing, and optimizes stimulation of the cyclic AMP receptors in chemotaxis and signal relaying.
Insights
The study investigated cyclic AMP phosphodiesterase in Dictyostelium discoideum, revealing non-Michaelian kinetics and a cryptic inhibitor binding site. This enzyme plays a key role in cellular slime mold aggregation and signaling.
Area of Science:
- Biochemistry
- Cell Biology
- Developmental Biology
Background:
- Cellular slime molds like Dictyostelium discoideum aggregate via chemotaxis.
- Cyclic AMP (cAMP) signaling is crucial for Dictyostelium discoideum aggregation.
- Membrane-bound enzymes play critical roles in cellular communication.
Purpose of the Study:
- To characterize the kinetics of membrane-bound cyclic AMP phosphodiesterase in Dictyostelium discoideum.
- To investigate the enzyme's specificity and its interaction with cAMP receptors.
- To understand the enzyme's role in the aggregation process.
Main Methods:
- Studied kinetics of particle-bound and cell-bound phosphodiesterase.
- Used external cyclic AMP as a substrate.
- Solubilized and partially purified the membrane-bound enzyme using lithium 3,5-diiodosalicylate.
Main Results:
- Both particle-bound and cell-bound phosphodiesterase exhibited non-Michaelian kinetics.
- The membrane-bound enzyme showed different specificity compared to the cAMP receptor.
- Solubilized enzyme retained non-linear kinetics but lost sensitivity to an inhibitor, indicating a cryptic binding site.
Conclusions:
- The cell-bound cyclic AMP phosphodiesterase has a cryptic inhibitor binding site.
- The enzyme's kinetics optimize stimulation of cAMP receptors during aggregation.
- This enzyme likely regulates the 'time window' for signal relaying in Dictyostelium discoideum.