Sophisticated regulation of the actin cytoskeleton by a variety of actin-binding proteins is essential for eukaryotic cells to perform their diverse functions. The plastin (also know, as fimbrin) protein family belongs to actin-bundling proteins, and the protein family is evolutionarily conserved and expressed in yeast, plant, and animal cells. Plastins are characterized by EF-hand Ca2+-binding domains and actin-binding domains and can cross-link actin filaments into higher-order assemblies like bundles. Three isoforms have been identified in mammals. T-plastin is expressed in cells from solid tissues, such as neurons in the brain. I-plastin expression is restricted to intestine and kidney; the isoform plays a vital role in the function of absorptive epithelia in these organs. L-plastin is expressed in hematopoietic cell lineages and in many types of cancer cells; the isoform is thus considered to be a useful biomarker for cancer. 1. Introduction Dynamics of the actin cytoskeleton is one of the cardinal features of eukaryotic cells, which is essential for fundamental cellular functions such as cell division, intracellular traffic of organelles, cell morphology, and cell motility [1]. The architecture of the actin cytoskeleton is regulated by a variety of proteins termed actin-binding proteins [1]. Actin filaments are organized into two types of arrays: bundles and weblike networks. Likewise, the actin filaments cross-linking proteins that help to stabilize and maintain these distinct structures are divided into two classes: bundling proteins and web-forming proteins. The plastin (also known as fimbrin) protein family belongs to bundling proteins and is evolutionarily conserved from yeast to mammalian cells. In mammals, three isoforms are known to be expressed in a cell-type-specific manner and exhibit distinct properties (Table 1 and Figure 1). In this paper, studies regarding the structure and biological functions of plastins are reviewed. Table 1: Plastin isoforms expressed in distinct cell types. Figure 1: Human and mouse plastin isoforms. The homology between the amino acid sequences of these isoforms is showed. 2. Structure of Plastins The structure of plastins is well conserved from lower eukaryotes to humans and is characterized by actin binding domains (ABD). An ABD consists of a pair of ~125 residue calponin-homology (CH) domains (Figure 2(a)). ABD-containing proteins include proteins such as spectrin, α-actinin, dystrophin, cortexillin, and plastin/fimbrin [2]. The plastins are unique among these, as they possess two tandem repeats of ABD
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