title: Pig platelet tropomyosin: interactions with the other thin-filament proteins.
abstract: Pig platelet tropomyosin exhibits many of the functional activities of skeletal tropomyosin. At low ionic strength it forms end-to-end aggregates similar to those formed by skeletal tropomyosins. It forms a 1:1 complex with muscle troponin or with a troponin I-pig brain calmodulin complex, as well as a 1:6 association with platelet filamentous actin. Electron microscopy of paracrystals shows that the troponin binding site is slightly C-terminal of the unique cysteine, corresponding to position 190 of the rabbit skeletal alpha-tropomyosin sequence. The effect of a complex comprising platelet actin and tropomyosin on the ATPase activity of rabbit skeletal muscle myosin subfragment-1 was similar to that displayed by its skeletal muscle counterpart. Platelet tropomyosin decreased the activity by roughly half in a calcium-independent manner. Addition of troponin to the actin-tropomyosin in the absence of calcium results in further inhibition and allows the full activity of the complex to be restored by Ca2+. These results differ from those obtained by Côté & Smillie for horse platelet tropomyosin and this may reflect the different isomeric nature of pig platelet tropomyosin. These results suggest that the functional properties of non-muscle tropomyosins may differ when comparisons are made between proteins isolated from the same type of cell but in different species. Differences in self-association and actin-binding properties may be finely graded between different isoforms.
mesh_terms: D000199:Actins / Q000302:isolation & purification / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D001792:Blood Platelets / Q000032:analysis*; D002118:Calcium / Q000378:metabolism; D046911:Macromolecular Substances; D008274:Magnesium; D008854:Microscopy, Electron; D009132:Muscles / Q000378:metabolism; D009994:Osmolar Concentration; D011817:Rabbits; D014335:Tropomyosin / Q000378:metabolism*; D014336:Troponin / Q000378:metabolism; D014783:Viscosity
chemical_list: D000199:Actins; D046911:Macromolecular Substances; D014335:Tropomyosin; D014336:Troponin; D000251:Adenosine Triphosphatases; D008274:Magnesium; D002118:Calcium