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Uptake of calcium ions into microsomes isolated from Physarum polycephalum
Abstract:
Membranous vesicles (microsomes) were isolated from plasmodia of the acellular slime mold, Physarum polycephalum. The microsomes were about 0.2 about 0.2 micronM in diameter, and about 10 nm thick. The main protein component of the vesicles had a molecular weight of 100,000 daltons. Calcium ions were taken up by the microsomes only in the presence of Mg2+- ATP. The maximum amount of Ca2+ ions accumulated in the microsomes was 0.24 micronmole/mg protein. The Ca2+ uptake was not accelerated by oxalate. The ATPase [EC 3.6.1.3] activity required Ca2+ ions for full activation. The concentration of Ca2+ ions required for half-maximum activation was about 1 micronM. The Km and Vm values were 53 micronM and 1.6 micronmole/(mg-min), respectively. About 0.2 mole of Ca2+ ions was taken up by the microsomes, coupled with the hydrolysis of 1 mole of ATP. THE ATPase activity and Ca2+ uptake of the microsomes were not inhibited by sodium azide. Furthermore, electron microscopic examination showed that mitochondrial contamination was slight. These results suggest that a vesicular calcium transport system, analogous to the sacroplasmic reticulum in skeletal muscle, is involved in regulation of the Ca2+ concentration in plasmodia of Physarum.
Insights
Physarum polycephalum microsomes actively transport calcium ions using Mg2+-ATP. This vesicular calcium transport system regulates intracellular calcium levels in slime mold plasmodia.
Area of Science:
- Cell Biology
- Biochemistry
Background:
- Acellular slime mold, Physarum polycephalum, exhibits complex cellular processes.
- Regulation of intracellular calcium (Ca2+) is crucial for cellular functions.
Purpose of the Study:
- To investigate the presence and function of a calcium transport system in Physarum polycephalum microsomes.
Main Methods:
- Isolation and characterization of membranous vesicles (microsomes) from Physarum polycephalum plasmodia.
- Measurement of Ca2+ uptake and ATPase activity in isolated microsomes.
- Electron microscopy to assess microsomal purity.
Main Results:
- Microsomes demonstrated Mg2+-ATP-dependent Ca2+ uptake, with a maximum accumulation of 0.24 µmole/mg protein.
- ATPase activity was Ca2+-dependent, with half-maximal activation at approximately 1 µM Ca2+.
- Ca2+ transport was coupled to ATP hydrolysis (0.2 Ca2+ / ATP), and unaffected by oxalate or azide.
Conclusions:
- Physarum polycephalum microsomes possess an active Ca2+ transport system.
- This system is analogous to the sarcoplasmic reticulum in muscle and likely regulates cytoplasmic Ca2+ concentrations.