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Irradiations of rabbit myofibrils with an ultraviolet microbeam. II. Phalloidin protects actin in solution but not in
Abstract:
We tested whether phalloidin protects actin in myofibrils from depolymerization by ultraviolet light (UV). I bands in glycerinated rabbit psoas myofibrils were irradiated with a UV microbeam in the presence and absence of phalloidin. We used the retention of contractility of the irradiated I band as the assay for protection of actin by phalloidin, since previous experiments indicated that UV blocks contraction of an irradiated I band by depolymerizing the thin filaments. The I bands of myofibrils incubated in phalloidin were as sensitive to UV as control I bands, indicating that phalloidin did not protect the thin filaments. However, phalloidin did protect F-actin in solution from depolymerization by UV. This apparent contradiction between F-actin in myofibrils and F-actin in solution was resolved by observing unirradiated myofibrils that were stained with rhodamine-phalloidin. It was found that phalloidin does not bind uniformly to the thin filaments, though as the fluorescence image is observed over time the staining pattern changes until it does appear to bind uniformly. We conclude that phalloidin does not protect F-actin in myofibrils from depolymerization by UV because it does not bind uniformly to the filaments.
Insights
Phalloidin does not protect muscle actin filaments from UV damage within myofibrils. This is because phalloidin does not bind uniformly to these actin structures, unlike in solution.
Area of Science:
- Muscle physiology
- Biochemistry
- Cell biology
Background:
- Actin filaments (F-actin) are crucial for muscle contraction.
- Ultraviolet (UV) light can depolymerize actin, inhibiting muscle function.
- Phalloidin is known to stabilize F-actin in solution.
Purpose of the Study:
- To investigate if phalloidin protects actin in myofibrils from UV-induced depolymerization.
- To understand the mechanism behind phalloidin's interaction with myofibrillar actin.
Main Methods:
- Irradiation of myofibril I-bands with a UV microbeam in the presence and absence of phalloidin.
- Assessing actin protection by measuring the retention of myofibril contractility.
- Observing phalloidin binding patterns in myofibrils using rhodamine-phalloidin staining.
Main Results:
- Phalloidin did not protect myofibrillar actin from UV-induced depolymerization.
- Myofibrils incubated with phalloidin showed similar UV sensitivity to controls.
- Phalloidin protected F-actin in solution from UV damage.
- Rhodamine-phalloidin staining revealed non-uniform binding of phalloidin to myofibrillar thin filaments.
Conclusions:
- Phalloidin fails to protect actin within myofibrils from UV depolymerization.
- The lack of protection is attributed to phalloidin's non-uniform binding to thin filaments in myofibrils.
- This contrasts with phalloidin's protective effect on F-actin in solution, highlighting differences in binding dynamics.