Amino terminal, but not the carboxy terminal, sequences of tropomyosin-1 are essential for the induction of stress fiber assembly in neoplastic cells

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

Human tropomyosin isoforms in the regulation of cytoskeleton functions

Over the past two decades, extensive molecular studies have identified multiple tropomyosin isoforms existing in all mammalian cells and tissues. In humans, tropomyosins are encoded by TPM1 (alpha-Tm, 15q22.1), TPM2 (beta-Tm, 9p13.2-p13.1), TPM3 (gamma-Tm, 1q21.2) and TPM4 (delta-Tm, 19p13.1) genes. Through the use of different promoters, alternatively spliced exons and different sites of poly(A) addition signals, at least 22 different tropomyosin cDNAs with full-length open reading frame have been cloned. Compelling evidence suggests that these isoforms play important determinants for actin cytoskeleton functions, such as intracellular vesicle movement, cell migration, cytokinesis, cell proliferation and apoptosis. In vitro biochemical studies and in vivo localization studies suggest that different tropomyosin isoforms have differences in their actin-binding properties and their effects on other actin-binding protein functions and thus, in their specification ofactin microfilaments. In this chapter, we will review what has been learned from experimental studies on human tropomyosin isoforms about the mechanisms for differential localization and functions of tropomyosin. First, we summarize current information concerning human tropomyosin isoforms and relate this to the functions of structural homologues in rodents. We will discuss general strategies for differential localization oftropomyosin isoforms, particularly focusing on differential protein turnover and differential isoform effects on other actin binding protein functions. We will then review tropomyosin functions in regulating cell motility and in modulating the anti-angiogenic activity of cleaved high molecular weight kininogen (HKa) and discuss future directions in this area.

DAP kinase mediates the phosphorylation of tropomyosin-1 downstream of the ERK pathway, which regulates the formation of stress fibers in response to oxidative stress

Endothelial cells are actively involved in regulating the exchanges between blood and tissues. This function is tightly dependent on actin cytoskeleton dynamics and is challenged by a wide variety of stimuli, including oxidative stress. In endothelial cells, oxidative stress quickly activates the extracellular-signal-regulated kinase (ERK) MAP kinase, which results in the phosphorylation of tropomyosin. Here, we investigated further the mechanisms of tropomyosin phosphorylation and its function in actin remodeling. We identified, for the first time, death-associated protein kinase 1 (DAP kinase 1) as the kinase that phosphorylates tropomyosin-1 in response to ERK activation by hydrogen peroxide (H(2)O(2)). We also report that the phosphorylation of tropomyosin-1 mediated by DAP kinase occurs on Ser283. Moreover, the expression of the pseudophosphorylated tropomyosin mutant Ser283Glu triggers by itself the formation of stress fibers in untreated cells, and the effect is maintained in H(2)O(2)-treated cells in which DAP kinase expression is knocked-down by siRNA. By contrast, the expression of the nonphosphorylatable tropomyosin mutant Ser283Ala is not associated with stress fibers and leads to membrane blebbing in response to H(2)O(2). Our finding that tropomyosin-1 is phosphorylated downstream of ERK and DAP kinase and that it helps regulate the formation of stress fibers will aid understanding the role of this protein in regulating the endothelial functions associated with cytoskeletal remodeling.