title: An actin-binding protein, LlLIM1, mediates calcium and hydrogen regulation of actin dynamics in pollen tubes.
abstract: Actin microfilaments are crucial for polar cell tip growth, and their configurations and dynamics are regulated by the actions of various actin-binding proteins (ABPs). We explored the function of a lily (Lilium longiflorum) pollen-enriched LIM domain-containing protein, LlLIM1, in regulating the actin dynamics in elongating pollen tube. Cytological and biochemical assays verified LlLIM1 functioning as an ABP, promoting filamentous actin (F-actin) bundle assembly and protecting F-actin against latrunculin B-mediated depolymerization. Overexpressed LlLIM1 significantly disturbed pollen tube growth and morphology, with multiple tubes protruding from one pollen grain and coaggregation of FM4-64-labeled vesicles and Golgi apparatuses at the subapex of the tube tip. Moderate expression of LlLIM1 induced an oscillatory formation of asterisk-shaped F-actin aggregates that oscillated with growth period but in different phases at the subapical region. These results suggest that the formation of LlLIM1-mediated overstabilized F-actin bundles interfered with endomembrane trafficking to result in growth retardation. Cosedimentation assays revealed that the binding affinity of LlLIM1 to F-actin was simultaneously regulated by both pH and Ca(2+): LlLIM1 showed a preference for F-actin binding under low pH and low Ca(2+) concentration. The potential functions of LlLIM1 as an ABP sensitive to pH and calcium in integrating endomembrane trafficking, oscillatory pH, and calcium circumstances to regulate tip-focused pollen tube growth are discussed.
authors.affiliation: Institute of Plant and Microbial Biology, Academia Sinica, Nankang, Taipei 115, Taiwan, Republic of China.
authors.forename: Huei-Jing
authors.initials: HJ
authors.lastname: Wang
authors.forename: Ai-Ru
authors.initials: AR
authors.lastname: Wan
authors.forename: Guang-Yuh
authors.initials: GY
authors.lastname: Jauh
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D000199:Actins / Q000378:metabolism*; D000595:Amino Acid Sequence; D002118:Calcium / Q000378:metabolism*; D003001:Cloning, Molecular; D018525:Germination; D006056:Golgi Apparatus / Q000502:physiology; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D027762:Lilium / Q000254:growth & development / Q000378:metabolism* / Q000648:ultrastructure; D008840:Microfilament Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D053205:Pollen Tube / Q000254:growth & development / Q000378:metabolism / Q000648:ultrastructure; D020033:Protein Isoforms / Q000737:chemistry / Q000378:metabolism / Q000502:physiology; D016415:Sequence Alignment; D015398:Signal Transduction; D022161:Transport Vesicles / Q000502:physiology
chemical_list: D000199:Actins; D008840:Microfilament Proteins; D010940:Plant Proteins; D020033:Protein Isoforms; D006859:Hydrogen; D002118:Calcium