Interaction of a Dictyostelium member of the plastin/fimbrin family with actin filaments and actin-myosin complexes.

Abstract

A protein purified from cytoskeletal fractions of Dictyostelium discoideum proved to be a member of the fimbrin/plastin family of actin-bundling proteins. Like other family members, this Ca2+-inhibited 67-kDa protein contains two EF hands followed by two actin-binding sites of the a-actinin/ 3-spectrin type. Dd plastin interacted selectively with actin isoforms: it bound to D. discoideum actin and to 0/y-actin from bovine spleen but not to a-actin from rabbit skeletal muscle. Immunofluorescence labeling of growth phase cells showed accumulation of Dd plastin in cortical structures associated with cell surface extensions. In the elongated, streaming cells of the early aggregation stage, Dd plastin was enriched in the front regions. To examine how the bundled actin filaments behave in myosin II-driven motility, complexes of F-actin and Dd plastin were bound to immobilized heavy meromyosin, and motility was started by photoactivating caged ATP. Actin filaments were immediately propelled out of bundles or even larger aggregates and moved on the myosin as separate filaments. This result shows that myosin can disperse an actin network when it acts as a motor and sheds light on the dynamics of proteinprotein interactions in the cortex of a motile cell where myosin II and Dd plastin are simultaneously present.

Roles of a Fimbrin and an α-Actinin-like Protein in Fission Yeast Cell Polarization and Cytokinesis

“…Although it is possible that the accumulation of Fim1p patches at the cell center obscures the detection of a contracting ring (as can happen when cells are stained for actin; see Balasubramanian et al, 1997), it is also possible that actin filaments are bundled too tightly by Fim1p to allow access by myosin, so that myosin must displace Fim1p as contraction begins. Such displacement has been observed with the Dictyostelium proteins in vitro (Prassler et al, 1997). Because ␣-actinins can bind (directly or indirectly) to membrane proteins (Critchley and Flood, 1999), Ain1p may help to link the medial ring to the plasma membrane during contraction.…”

“…Use of this fragment to probe a genomic-DNA library yielded a plasmid containing a complete fimbrin coding sequence (here designated fim1) interrupted by two introns, as confirmed by the analysis of cDNA sequences (see MATERIALS AND METHODS). Sequences subsequently released by the genome project (cosmid c1778 on chromosome II) were identical within the fim1 coding region.The predicted Fim1p sequence of 614 amino acids is very similar (64, 49, and 42% identical, respectively, over the full lengths of the proteins) to those of S. cerevisiae Sac6p (Adams et al, 1991), Dictyostelium fimbrin (Prassler et al, 1997), and human L-plastin (Lin et al, 1988(Lin et al, , 1990 (Figure 2B). Like these related proteins, Fim1p has two apparent actin-binding motifs that are 41% identical to each other.…”

“…The predicted Fim1p sequence of 614 amino acids is very similar (64, 49, and 42% identical, respectively, over the full lengths of the proteins) to those of S. cerevisiae Sac6p (Adams et al, 1991), Dictyostelium fimbrin (Prassler et al, 1997), and human L-plastin (Lin et al, 1988(Lin et al, , 1990 (Figure 2B). Like these related proteins, Fim1p has two apparent actin-binding motifs that are 41% identical to each other.…”

“…Although ␣-actinins concentrate in the cleavage furrows of some animal cells and a fimbrin concentrates in the cleavage furrow of Tetrahymena (see INTRODUCTION), no strong evidence has been obtained to support direct roles for these proteins in cytokinesis in these cell types. Moreover, in Dictyostelium, neither ␣-actinin nor fimbrin is detectably enriched in the cleavage furrow, and no substantial cytokinesis defect has been detected in cells that are lacking either protein (Schleicher et al, 1988;Prassler et al, 1997;Rivero et al, 1999;Weber et al, 1999). Instead, two other actin-bundling proteins, the cortexillins, are enriched in cleavage furrows and required for normal cytokinesis (Faix et al, 1996;Weber et al, 1999).…”

“…Moreover, the mutant cells display defective actin cables and aberrant, mislocalized actin patches (Adams et al, 1991). Dictyostelium fimbrin also localizes to certain actin-rich regions of the cell (Prassler et al, 1997), but a mutant lacking fimbrin has no detectable defect in cytokinesis and is capable of completing development (Prassler and Gerisch, personal communication). Thus, fimbrin function may also overlap with that of other actinbinding proteins.…”