Morphology and Viscoelasticity of Actin Networks Formed with the Mutually Interacting Crosslinkers: Palladin and Alpha-actinin

Abstract

Actin filaments and associated actin binding proteins play an essential role in governing the mechanical properties of eukaryotic cells. Even though cells have multiple actin binding proteins (ABPs) that exist simultaneously to maintain the structural and mechanical integrity of the cellular cytoskeleton, how these proteins work together to determine the properties of actin networks is not clearly understood. The ABP, palladin, is essential for the maintenance of cell morphology and the regulation of cell movement. Palladin coexists with -actinin in stress fibers and focal adhesions and binds to both actin and -actinin. To obtain insight into how mutually interacting actin crosslinking proteins modulate the properties of actin networks, we characterized the micro-structure and mechanics of actin networks crosslinked with palladin and -actinin. We first showed that palladin crosslinks actin filaments into bundled networks which are viscoelastic in nature. Our studies also showed that composite networks of -actinin/palladin/actin behave very similar to pure palladin or pure -actinin networks. However, we found evidence that palladin and -actinin synergistically modify network viscoelasticity. To our knowledge, this is the first quantitative characterization of the physical properties of actin networks crosslinked with two mutually interacting crosslinkers.

Autophagy is required for ectoplasmic specialization assembly in sertoli cells

“…F-actin is packed in hexagonal arrays in the ES, while it appears as a branched network surrounding the tubular portion of TBCs. 43,55 As PDLIM1 could work as a scaffold to recruit other molecules to bind the F-actin bundles, and the binding of PDLIM1 to ACTN1 and PALLD may offer more actin binding site in a smaller volume, facilitating the F-actin interconnectivity, 9,60,61 so the accumulation of PDLIM1 may facilitate the branched F-actin network formation in TBCs rather than affecting their assembly. The disordered distribution of TBCs might be caused by the abnormal structure of apical ES or malformed sperm head, and these possibilities still need further experimental data to be distinguished in the future.…”

Actin‐induced dimerization of palladin promotes actin‐bundling

“…Filamin, for example, contains more linker domains than α‐actinin and this results in actin filament structures that are characterized by actin bundles that have more viscoelasticity than those formed by α‐actinin . The viscoelastic properties of actin filament structures formed by the isolated Ig3 and Ig3–4 domains have not been studied yet, however, the rheological properties of full length palladin‐actin networks have been found to be similar to those formed by α‐actinin . Conventional wisdom would suggest that since actin binding and dimerization sites reside in the same domain (Ig3), then palladin should form a stiffer actin network than those formed by α‐actinin.…”

Actin binding proteins, spermatid transport and spermiation

“…These changes in the organization of the actin cytoskeleton thus confers plasticity to cells besides mechanical integrity, which also induces changes in the localization of the adhesion protein complexes at the TJ, such as occludin-ZO-1(zonula occludens-1) and claudin-ZO-1, apical ES such as integrin-laminin and nectin-afadin, which all use F-actin for their attachment [44]. Actin re-organization also enable cells to carry out various functions, including cell division, motility, contraction, phagocytosis and endocytic vesicle-mediated protein trafficing, besides conferring cell shape, polarity, adhesion and signal transduction [46,52]. Actin dynamics are tightly regulated by over 150 actin binding proteins (ABPs) that modulate localization, polymerization, cleavage, cross-linking, and organization of microfilaments, and they can be classified into two broad groups.…”

“…Palladin modulates the morphology and viscoelastic response of actin network in a concentration-dependent manner. Increasing palladin concentrations cause structural transitions in actin network from a weakly cross-linked phase to a strongly bundled phase in which branched bundles span the entire network [52]. The Ig domain also binds directly to other actin binding proteins, such as ezrin (component of the cortical actin cytoskeleton) and α-actinin [84–86,89].…”

“…α-Actinin is also an actin cross-linking protein, and also a scaffold that recruits other signaling molecules to the cytoskeleton. Binding of palladin to α-actinin may offer more actin binding sites in a smaller volume, enabling the formation of tighter actin bundles, and facilitating bundle interconnectivity [52]. The ability of palladin to connect other molecules suggests its role as a cytoskeletal scaffold that brings other proteins with different functional modalities into higher-order molecular complexes [87].…”