Biophysical Chemistry 24 (1986) 211-215 Elsevier 211 BPC 01070 DIFFERENCES IN THERMAL STABILITY OF FROG AND RABBIT aa- AND a/?-TROPOMYOSLNS DETERMINED BY OPTICAL ROTATORY DISPERSION Ssren HVIDT Depur~ments of Chemistry, Rise Nahonai Laborafory and Roskilde Unisersrry, DK-4000 Roskilde, Denmark Received 18th December 1985 Revised manuscript received 10th April 1986 Accepted 2nd May 1986 Key words: Optical rotatory dispemon; Helix- coil transirio!t; Thermal stability; a- Trop0myo.h; p - Tropomyosin; (Frog) Frog and rabbit an- and n/%tropomyosins were purified. and their thermal stabilities determined hy use of optical rotatory dispersion. The tropomyosins were found to he virtually completely helical at 5°C. Regions of different thermal stahihties were seen for all tropomyosins. Rabbit and frog acr-tropornyosin show very similar thermal properties, with main transitions near 47-49OC. The main transition for frog c&tropomyosm is at 32OC. The results show that the olfl-tropomyosins are less stable than the ala-forms. Only thermal transitions of the &forms appear to he correlated with the body temperatures of the animals. 1. Introduction Tropomyosin (TM) is an important component of the contractile apparatus of skeletal and cardiac muscles, where it is involved in the calcium-depen- dent regulation of myosin-actin interactions to- gether with troponin [l]. TM is also found in several nonmuscle cells [2], where its function is at present unclear. Rabbit TM consists of two 284- residue peptide chains, arranged in register and parallel, forming a coiled-coil a-helical structure [l]. Skeletal muscle tissues contain two major forms of TM chains, designated as LY-and B-chains. Most tissues contain (YCK- and a&TMs, in propor- tions depending on the type and source of muscle [3]. The physiological significance of the two TM forms remains unknown, and little is known about physicochemical and biochemical differences be- sides their amino acid sequences [4] and degrees of phosphorylation [ 51. The thermal stability of rabbit CW-TM has been studied extensively by use of a broad range of techniques including circular dichroism and opti- cal rotatory dispersion (ORD) [6], fluorescence [7], calorimetry [8] and enzyme probes [9]. Most of these studies support the idea that rabbit TM contains regions or domains with different ther- mal stabilities. Less stable domains could have physiological significance in terms of giving flexi- bility to the structure or being involved in confor- mational transitions during regulation [6610]. Only few studies have been reported on the stabilities of TMs from other species and &forms [lo-121. Therefore, thermal stabilities, as monitored by ORD, were determined of WY- and &TMs from frog and rabbit, in order to investigate whether CW- and &TM, show different thermal stabilities and to see whether thermal transitions are corre- lated with the body temperatures of the animals. 2. Materials and methods Rabbit and frog (Rana temporaria) TMs were extracted and purified from ethanol and acetone- 0301-4622/W/$03.50 0 1986 Elsevier Science Publishers B.V. (Biomedical Division)