The subunits and biological activity of polymorphic forms of tropomyosin

Abstract

1. Free thiol groups were shown to be essential for tropomyosin to effect maximum inhibition of the Ca(2+)-stimulated ATPase (adenosine triphosphatase) of desensitized actomyosin but not for its activity in the regulatory-protein system. 2. The activity of tropomyosin on the Mg(2+)-stimulated ATPase in the regulatory-protein system was more susceptible to enzymic digestion and thermal denaturation than its effect on the Ca(2+)-stimulated ATPase of actomyosin. 3. Rabbit skeletal tropomyosin migrated as two distinct electrophoretic components in the presence of sodium dodecyl sulphate and urea and as four components on isoelectric focusing in urea. 4. The two main subunits present in rabbit skeletal tropomyosin, which have been named the alpha- and beta-chains, were separated by chromatography on CM-cellulose in urea at pH4.0. They were shown to be virtually identical in amino acid composition, except for their cysteine contents. The alpha(2) and beta(2) forms of tropomyosin possessed all the biological activities characteristic of normal tropomyosin preparations. 5. In skeletal muscle the alpha and beta components of tropomyosin were present in the proportion of 4:1. Somewhat lower ratios were obtained in skeletal muscle of sheep, pig and cow. 6. Tropomyosin isolated from cardiac muscle and Pecten maximus adductor muscle migrated as one band only. These tropomyosins possessed similar biological activities to those isolated from skeletal muscle.

Differential distribution of tropomyosin subunits in fast and slow rat muscles and its changes in long‐term denervated hemidiaphragm

“…The same splitting of the tropomyosin spots is present occasionally in some slabs, but without any consistent difference among muscles, so that it can result from artifacts during isoelectrofocussing. On the other hand microheterogeneity [4] or phosphorylation [ 141 of tropomyosin subunits have been reported; so it remains to be elucidated whether the single spot pattern or the splitting is representative of the in vivo condition. As a consequence of this uncertainty, we consider the ratio between (Y and 0 subunit in table 1.…”

A new method of preparation of troponin I (inhibitory protein) using affinity chromatography. Evidence for three different forms of troponin I in striated muscle

“…When tested against desensitized actomyosin of rabbit white skeletal muscle using tropomyosln from the same muscle (consisting of 80% (Y subunit, and of 20% fl subunit [6,151) the inhibitory activities of the troponin I preparations were in the order white skeletal > red skeletal > cardiac muscle ( fig. 3).…”

“…Desensitized actomyosin and tropomyosin were prepared by the procedures described by Cummins and Perry [6].…”

Tropomyosin: Evidence for no stagger between chains

“…Troponin-free tropomyosin was prepared from rabbit back and thigh muscles as described by Cummins and Perry [2] and its concentration measured by ultraviolet absorption taking E!$* = 2.9 cm-' [4]. Oxidised tropomyosin was prepared by stirring tropomyosin in 1 M NaCl, 25 mM sodium borate, North-Holland Publishing Company -Amsterdam 25 mM CuClz pH 9.3 at 20°C under air until no free sulphydryl groups could be detected.…”

“…Chemical analysis of rabbit skeletal tropomyosin has indicated two types of chain, designated Q and 0, with the cx being about four times as abundant [2]. These chains have similar amino-acid compositions [2] and sequences [3] , but they differ in cysteine content with the a-chain have one such residue compared with two for the fl [2] . A pseudo-repeat of 7 residues has been demonstrated in the sequence, which produces a strip of hydrophobic residues down one side of each chain [3] .…”