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Low Mr tropomyosin isoforms from chicken brain and intestinal epithelium have distinct actin-binding properties
Abstract:
Tropomyosin isoforms of the low Mr class were isolated from chicken intestinal epithelium and brain, and their physical and functional properties were characterized. Tropomyosin from each tissue contains four distinct polypeptides, all of about 32,000 daltons. In two-dimensional gels, brain tropomyosin contains two major and two minor polypeptides; the major epithelium isoforms coelectrophorese with the two minor brain isoforms. Conversely, only small amounts of the major brain isoforms are detected in the epithelium. Actin-binding properties of brain tropomyosin isoforms are distinct from those of the intestinal epithelium. At 2.5 mM MgCl2 and physiological ionic strength, the intestinal epithelial tropomyosin binds to filamentous actin with an apparent Ka of 8 X 10(6) M-1 whereas brain tropomyosin has an apparent Ka of 8 X 10(5) M-1. Tropomyosin from either tissue binds actin cooperatively with a Hill coefficient of 2.3 for intestinal epithelial cell and 1.95 for brain tropomyosin. Isoforms from both tissues exhibit reduced head-to-tail polymerizability as compared to muscle tropomyosin. The actin-binding properties of intestinal epithelial cell tropomyosin are therefore similar to those of the muscle tropomyosins even though the isoforms have lower molecular weight, a paracrystal structure, and reduced head-to-tail polymerizability typical of the other nonmuscle tropomyosins. These results indicate that a heterogeneity of functional properties may be expressed among the low Mr tropomyosin isoforms.
Insights
Chicken tropomyosin isoforms from the intestine and brain exhibit distinct actin-binding properties. These low molecular weight (Mr) tropomyosin isoforms show functional heterogeneity, impacting cellular processes.
Area of Science:
- Biochemistry
- Cell Biology
- Muscle Physiology
Background:
- Tropomyosin (TM) is a crucial protein involved in muscle contraction and non-muscle cellular functions.
- Low molecular weight (Mr) tropomyosin isoforms are found in various non-muscle tissues, but their functional diversity is not fully understood.
- Understanding tissue-specific tropomyosin properties is essential for elucidating their roles in cellular mechanics.
Purpose of the Study:
- To isolate and characterize low Mr tropomyosin isoforms from chicken intestinal epithelium and brain.
- To compare the physical and functional properties, particularly actin-binding characteristics, of these tropomyosin isoforms.
- To investigate the potential functional heterogeneity among non-muscle tropomyosin isoforms.
Main Methods:
- Isolation of low Mr tropomyosin isoforms from chicken intestinal epithelium and brain.
- Characterization of polypeptide composition using two-dimensional gel electrophoresis.
- Assessment of actin-binding affinity (apparent Ka) and cooperativity (Hill coefficient) using filamentous actin.
- Evaluation of head-to-tail polymerizability compared to muscle tropomyosin.
Main Results:
- Both tissues yielded four distinct tropomyosin polypeptides of approximately 32,000 daltons.
- Brain tropomyosin contained two major and two minor polypeptides, with major isoforms differing from intestinal epithelial isoforms.
- Intestinal epithelial tropomyosin exhibited a higher apparent actin-binding affinity (Ka = 8 x 10^6 M-1) than brain tropomyosin (Ka = 8 x 10^5 M-1).
- Both isoforms bound actin cooperatively, with Hill coefficients of 2.3 (epithelium) and 1.95 (brain).
- Both tissue tropomyosins showed reduced head-to-tail polymerizability compared to muscle tropomyosin.
Conclusions:
- Chicken intestinal epithelium and brain express distinct low Mr tropomyosin isoforms with differing functional properties.
- Despite lower molecular weight and reduced polymerizability, intestinal epithelial tropomyosin displays actin-binding characteristics similar to muscle tropomyosins.
- Functional heterogeneity exists among low Mr tropomyosin isoforms, suggesting specialized roles in different non-muscle tissues.