Isolation from Thyroid Cells of Purified Plasma Membranes with Associated Actin Microfilaments

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Abstract
Plasma membranes of thyroid cells were purified from hog thyroid glands following 2 procedures. Their homogeneity was tested by EM and by measurements of the activity of membrane-bound enzyme markers. According to the procedure used the membrane fractions obtained present some differences in their morphological features as well as in the repartition of the activities of the membrane-bound enzyme markers. Whatever the composition of the membrane fraction examined (membrane vesicles, single membrane sheets with junctional complexes), decoration with heavy meromyosin clearly shows the presence of actin filaments attached to these fragments. Analysis of proteins by polyacrylamide gel electrophoresis indicates the presence of about 12 major components with actin. Treatment of membranes with Triton X-100 results in an insoluble core which contains all the actin and most of the major proteins. The selective extraction of these components by buffers differing in their ionic strength, pH, or the presence or absence of ATP.cntdot.Mg was used to characterize some of the proteins associated to actin; among them are filamin, myosin, .alpha.-actinin, tropomyosin.