Tropomyosin is a tetramer under physiological salt conditions.

Tropomyosin (TM) is a coiled-coil dimer of alpha-helical peptides, which self associates in a head- to-tail fashion along actin polymers, conferring stability to the microfilaments and serving a regulatory function in acto-myosin driven force generation. While the major amount of TM is associated with filaments also in non-muscle cells, it was recently reported that there are isoform-specific pools of TM multimers (not associated with F-actin), which appear to be utilized during actin polymerization and reformed during depolymerization. To determine the size of these multimers, skeletal muscle TM was studied under different salt conditions using gel-filtration and sucrose gradient sedimentation, and compared with purified non-muscle TM 1 and 5, as well as with TM present in non-muscle cell extracts and skeletal muscle TM added to such extracts. Under physiological salt conditions TM appears as a single homogenous peak with the Stokes radius 8.2 nm and the molecular weight (mw) 130,000. The corresponding values for TM 5 are 7.7 nm and 104,000, respectively. This equals four peptides, implying that native TM is a tetramer in physiological salt. It is therefore concluded that the TM multimers are tetramers.

Tropomyosin assembly intermediates in the control of microfilament system turnover.

Tropomyosin is a coiled-coil alpha-helical protein, which self-associates in a head-to-tail fashion along polymers of actin to produce thin filaments. Mammalian non-muscle cells express a large number of tropomyosin isoforms, which are differentially regulated during embryogenesis and associated with specialized actin microfilament ensembles in cells. The function of tropomyosin in specifying form and localization of these subcellular structures, and the precise mechanism(s) by which they carry out their functions, is unclear. This paper reports that, while the major fraction of non-muscle cell tropomyosin resides in actin thin filaments of the cytomatrix, the soluble part of the cytoplasm contains tropomyosins in the form of actin-free multimers, which are isoform specific and of high molecular weight (MW(app) 180,000-250,000). Stimulation of motile cells with growth factors induces a rapid, actin polymerization-dependent outgrowth of lamellipodia and filopodia. Concomitantly, the levels of tropomyosin isoform-specific multimers decrease, suggesting their involvement in actin thin filament formation. Malignant tumor cells have drastically altered levels and composition of tropomyosin isoform-specific multimers as well as tropomyosin in the cytomatrix.

Tropomyosins regulate the impact of actin binding proteins on actin filaments.

The state of actin depends intimately on its interaction partners in eukaryotic cells. Classically, the cooperative force-generating acto-myosin couple is turned off and on by the calcium-dependent binding and release of tropomyosin molecules. The situation with nonmuscle cells appears to be much more complicated, with tropomyosin isoforms regulating the kinds of tension-producing and stress-bearing structures formed of actin filaments. The polymerization of even the shortest gelsolin-capped filaments is efficiently promoted by the binding of tropomyosin, for example, a process that might occur all the way out to the leading edges of advancing cells. Recently, multimers of tropomyosin have been discovered that appear to be assembly intermediates, formed from identical tropomyosin molecules, which act as ready pools of tropomyosin during the catalytic formation of lamellipodia and filopodia. Remarkably, these multimers apparently reform during the disassembly of cellular actin-containing structures. The existence of these recyclable, tropomyosin isoform-specific structures suggests how cells prevent nonproductive association of non-identical, but closely similar, tropomyosin isoforms.

Tropomyosins are present in lamellipodia of motile cells.

This paper shows that high-molecular-weight tropomyosins (TMs), as well as shorter isoforms of this protein, are present in significant amounts in lamellipodia and filopodia of spreading normal and transformed cells. The presence of TM in these locales was ascertained by staining of cells with antibodies reacting with endogenous TMs and through the expression of hemaglutinin- and green fluorescent protein-tagged TM isoforms. The observations are contrary to recent reports suggesting the absence of TMs in regions,where polymerization of actin takes place, and indicate that the view of the role of TM in the formation of actin filaments needs to be significantly revised.