Modulation of actin mechanics by caldesmon and tropomyosin

Abstract

The ability of cells to sense and respond to physiological forces relies on the actin cytoskeleton, a dynamic structure that can directly convert forces into biochemical signals. Because of the association of muscle actin-binding proteins (ABPs) may affect F-actin and hence cytoskeleton mechanics, we investigated the effects of several ABPs on the mechanical properties of the actin filaments. The structural interactions between ABPs and helical actin filaments can vary between interstrand interactions that bridge azimuthally adjacent actin monomers between filament strands (i.e. by molecular stapling as proposed for caldesmon) or, intrastrand interactions that reinforce axially adjacent actin monomers along strands (i.e. as in the interaction of tropomyosin with actin). Here, we analyzed thermally driven fluctuations in actin's shape to measure the flexural rigidity of actin filaments with different ABPs bound. We show that the binding of phalloidin increases the persistence length of actin by 1.9-fold. Similarly, the intrastrand reinforcement by smooth and skeletal muscle tropomyosins increases the persistence length 1.5- and 2- fold respectively. We also show that the interstrand crosslinking by the C-terminal actin-binding fragment of caldesmon, H32K, increases persistence length by 1.6-fold. While still remaining bound to actin, phosphorylation of H32K by ERK abolishes the molecular staple (Foster et al. 2004. J Biol Chem 279;53387-53394) and reduces filament rigidity to that of actin with no ABPs bound. Lastly, we show that the effect of binding both smooth muscle tropomyosin and H32K is not additive. The combination of structural and mechanical studies on ABP-actin interactions will help provide information about the biophysical mechanism of force transduction in cells.

p53 Suppresses Src-Induced Podosome and Rosette Formation and Cellular Invasiveness through the Upregulation of Caldesmon

“…Caldesmon is a likely candidate that is known to stabilize actin stress fibers and regulate actomyosin-mediated contractility (23,26). It has been shown recently to be a potent suppressor of podosome formation (14,24) and, like p53, is downregulated in many cancer cells (43,47,49).…”

“…Caldesmon is a calmodulin-and actin-binding protein (48) whose roles in modulating the dynamics of the actin cytoskeleton and contractility during cell movement are well documented (23,26). We and others (14,24,39) have recently shown that caldesmon negatively regulates the formation of podosomes and rosettes by vascular smooth muscle cells (SMC) transformed with Src.…”

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

“…Tropomyosin is another component of the actin cytoskeleton in the hyphal tip. This protein stabilizes F-actin and controls F-actin mechanics, regulating the interaction between F-actin and myosins (215)(216)(217).…”

p53: Guardian of the genome is also a suppressor of cell invasion?

“…A survey of DNA sequences recognized by p53 reveals that the promoter site of caldesmon, an actin-binding protein found in podosomes, contains the consensus recognition site by p53 (unpublished data). Caldesmon is a calmodulin-and actinbinding protein 7 that is known to inhibit acto-myosin ATPase activity and cell contractility, stabilize actin stress fibers and destabilize Arp2/3-mediated actin nucleation. 8 Caldesmon was first shown to be a component of podosomes in 1993, 9 and more recently, it has been shown to play an antagonistic role in podosome formation 10,11 and its expression is downregulated in cancer cells.…”