Filamin-regulated F-actin Assembly Is Essential for Morphogenesis and Controls Phototaxis in Dictyostelium

Abstract

Dictyostelium strains lacking the F-actin cross-linking protein filamin (ddFLN) have a severe phototaxis defect at the multicellular slug stage. Filamins are rod-shaped homodimers that cross-link the actin cytoskeleton into highly viscous, orthogonal networks. Each monomer chain of filamin is comprised of an F-actin-binding domain and a rod domain. In rescue experiments only intact filamin re-established correct phototaxis in filamin minus mutants, whereas C-terminally truncated filamin proteins that had lost the dimerization domain and molecules lacking internal repeats but retaining the dimerization domain did not rescue the phototaxis defect. Deletion of individual rod repeats also changed their subcellular localization, and mutant filamins in general were less enriched at the cell cortex as compared with the full-length protein and were increasingly present in the cytoplasm. For correct phototaxis ddFLN is only required at the tip of the slug because expression under control of the cell type-specific extracellular-matrix protein A (ecmA) promoter and mixing experiments with wild type cells supported phototactic orientation. Likewise, in chimeric slugs wild type cells were primarily found at the tip of the slug, which acts as an organizer in Dictyostelium morphogenesis.

The filamins

“…61 In mice, complete loss of Flna expression causes embryonic lethality with severe defects in cardiovascular formation and bone development. [62][63][64] Mutations or depletions of FLN orthologs in other organisms, such as Dictyostelium, 65,66 Drosophila and C. elegans, 67 also result in developmental defects. Since numerous binding partners interact with FLNs ( Table 1 and Sup.…”

Filamin structure, function and mechanics: are altered filamin-mediated force responses associated with human disease?

“…The dimerisation domain is required to rescue a defective chemotaxis phenotype in dictyostelium FLN (ddFLN) null mutants (Khaire et al 2007), and a truncating mutation within human FLNC repeat 24 is associated with myofibrillar myopathy (Vorgerd et al 2005) owing to protein aggregation from reduced dimerisation and stability (Lowe et al 2007). The issue of whether FLNs can heterodimerise is not fully resolved; FLNB/C repeat 24 heterodimers were observed by in vitro cross-linking assays, but the combinations of FLNA/B or FLNA/C were not (Himmel et al Fig.…”

Cell type-specific filamin complex regulation by a novel class of HECT ubiquitin ligase is required for normal cell motility and patterning

“…Filamin is essential for slug phototaxis and filamin-deficient strains exhibit severe phototaxis defects (Fisher et al, 1997;Khaire et al, 2007;Knuth et al, 2004). It is thought that filamin is present in a 'phototaxis complex' that is involved in sensing light and in signal transduction at the tip of the slug (BandalaSanchez et al, 2006).…”

“…In Dictyostelium, filamin has been shown to play roles in the organisation of the actin cytoskeleton, in pseudopod formation and in the speed of movement of single cells. During development, however, the major role of filamin appears to be in slug phototaxis (Fisher et al, 1997;Khaire et al, 2007), which requires repeats 2-6 (Annesley et al, 2007). Filamin has been shown to function in the tip of the slug, in the pstA region, as expression of full-length filamin under the control of a pstA-specific promoter is able to completely rescue the Fln-null phototaxis phenotype (Khaire et al, 2007).…”

“…During development, however, the major role of filamin appears to be in slug phototaxis (Fisher et al, 1997;Khaire et al, 2007), which requires repeats 2-6 (Annesley et al, 2007). Filamin has been shown to function in the tip of the slug, in the pstA region, as expression of full-length filamin under the control of a pstA-specific promoter is able to completely rescue the Fln-null phototaxis phenotype (Khaire et al, 2007). Filamin has been proposed to act as an adaptor protein at the front of the slug during this process, as it is found within a large signalling complex that comprises many proteins, including RasD, ERKB, GRP125 and PKB (Bandala-Sanchez et al, 2006).…”