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Phosphatidylinositol 4,5-bisphosphate formation in rabbit skeletal and heart muscle membranes
Abstract:
Incubation of rabbit skeletal muscle microsomes or isolated triads with gamma 32P-ATP/Mg2+ in the absence and in the presence of added phosphatidylinositol resulted in the formation of phosphatidylinositol 4-phosphate catalyzed by phosphatidylinositol kinase. When phosphatidylinositol 4-phosphate was added as exogenous substrate, phosphatidylinositol 4,5-bisphosphate was also formed demonstrating the presence of a membrane bound phosphatidylinositol 4-phosphate kinase. Triads were broken mechanically in a French press and separated on a continuous sucrose gradient. Incubation of these fractions with gamma 32P-ATP/Mg2+ resulted in a rapid labeling of phospholipid in a membrane fraction banding between transverse tubules and the terminal cisternae. Partial triad breakage and triad reformation experiments indicated that this phosphatidylinositol kinase was associated with T-tubules. When exogenous phosphatidylinositol 4-phosphate was employed as substrate phosphatidylinositol 4,5-bisphosphate and phosphatidic acid were formed, indicating the presence of all the enzymes of the polyphosphoinositide signaling system in this special membrane fraction. In contrast, heart muscle microsomes or plasma membranes can catalyze this reaction sequence from endogenous formed phosphatidylinositol 4-phosphate.
Insights
Rabbit skeletal muscle triads contain enzymes for polyphosphoinositide signaling, particularly in T-tubules. These enzymes convert phosphatidylinositol to phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate.
Area of Science:
- Biochemistry
- Cell Biology
- Muscle Physiology
Background:
- Phosphoinositides are crucial signaling molecules in cellular processes.
- Skeletal muscle triads, the junctional interface between transverse tubules and sarcoplasmic reticulum, are vital for excitation-contraction coupling.
- The enzymatic machinery for polyphosphoinositide metabolism in skeletal muscle triads is not fully elucidated.
Purpose of the Study:
- To investigate the presence and localization of enzymes involved in polyphosphoinositide synthesis within rabbit skeletal muscle triads.
- To characterize the enzymatic activity of phosphatidylinositol kinase and phosphatidylinositol 4-phosphate kinase in isolated triads and their fractions.
Main Methods:
- Incubation of rabbit skeletal muscle microsomes and isolated triads with gamma 32P-ATP/Mg2+.
- Mechanical disruption of triads using a French press followed by sucrose gradient separation.
- Assay of enzymatic activity using exogenous and endogenous substrates like phosphatidylinositol and phosphatidylinositol 4-phosphate.
Main Results:
- Phosphatidylinositol kinase activity was detected, converting phosphatidylinositol to phosphatidylinositol 4-phosphate in triads.
- Phosphatidylinositol 4-phosphate kinase activity was also present, forming phosphatidylinositol 4,5-bisphosphate.
- Enzymatic activity was localized to a membrane fraction associated with T-tubules within the triad structure.
- All enzymes of the polyphosphoinositide signaling system were identified in this specialized T-tubule membrane fraction.
Conclusions:
- Rabbit skeletal muscle triads possess the complete enzymatic machinery for polyphosphoinositide synthesis, localized to the T-tubule membranes.
- This finding suggests a potential role for polyphosphoinositides in skeletal muscle excitation-contraction coupling.
- The identified enzymes are distinct from those found in heart muscle membranes, highlighting tissue-specific differences.