title: Sensing actin dynamics: structural basis for G-actin-sensitive nuclear import of MAL.
abstract: The coordination of cytoskeletal actin dynamics with gene expression reprogramming is emerging as a crucial mechanism to control diverse cellular processes, including cell migration, differentiation and neuronal circuit assembly. The actin-binding transcriptional coactivator MAL (also known as MRTF-A/MKL1/BSAC) senses G-actin concentration and transduces Rho GTPase signals to serum response factor (SRF). MAL rapidly shuttles between the cytoplasm and the nucleus in unstimulated cells but Rho-induced depletion of G-actin leads to MAL nuclear accumulation and activation of transcription of SRF:MAL-target genes. Although the molecular and structural basis of actin-regulated nucleocytoplasmic shuttling of MAL is not understood fully, it is proposed that nuclear import of MAL is mediated by importin α/β heterodimer, and that G-actin competes with importin α/β for the binding to MAL. Here we present structural, biochemical and cell biological evidence that MAL has a classical bipartite nuclear localization signal (NLS) in the N-terminal 'RPEL' domain containing Arg-Pro-X-X-X-Glu-Leu (RPEL) motifs. The NLS residues of MAL adopt an extended conformation and bind along the surface groove of importin-α, interacting with the major- and minor-NLS binding sites. We also present a crystal structure of wild-type MAL RPEL domain in complex with five G-actins. Comparison of the importin-α- and actin-complexes revealed that the binding of G-actins to MAL is associated with folding of NLS residues into a helical conformation that is inappropriate for importin-α recognition.
authors.affiliation: Structural Biology Research Center, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan.
authors.forename: Hidemi
authors.initials: H
authors.lastname: Hirano
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Matsuura
mesh_terms: D000199:Actins / Q000737:chemistry* / Q000378:metabolism; D021581:Active Transport, Cell Nucleus; D000818:Animals; D002467:Cell Nucleus / Q000378:metabolism*; D018360:Crystallography, X-Ray; D006801:Humans; D051379:Mice; D009154:Mutation; D041681:NIH 3T3 Cells; D019913:Nuclear Localization Signals / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary; D011817:Rabbits; D015534:Trans-Activators / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D028901:alpha Karyopherins / Q000737:chemistry / Q000378:metabolism
chemical_list: D000199:Actins; C511173:Mrtfa protein, mouse; D019913:Nuclear Localization Signals; D015534:Trans-Activators; D028901:alpha Karyopherins; C087978:karyopherin alpha 2