动植物中RNA结合蛋白的研究进展

RNA结合蛋白(RNA-binding proteins)在转录后基因表达调节中起着重要的作用,它通过和RNA相互作用来调节细胞的功能。RNA结合蛋白参与RNA剪接、多聚腺苷化作用、序列编辑、RNA转运、维持RNA的稳定和降解、细胞内定位和翻译控制等RNA代谢的各个方面。主要介绍了RNA结合蛋白的结构、靶标RNA及RNA结合蛋白在动植物和疾病中的研究。     

Nuclear RNP complex assembly initiates cytoplasmic RNA localization

Cytoplasmic localization of mRNAs is a widespread mechanism for generating cell polarity and can provide the basis for patterning during embryonic development. A prominent example of this is localization of maternal mRNAs in Xenopus oocytes, a process requiring recognition of essential RNA sequences by protein components of the localization machinery. However, it is not yet clear how and when such protein factors associate with localized RNAs to carry out RNA transport. To trace the RNA-protein interactions that mediate RNA localization, we analyzed RNP complexes from the nucleus and cytoplasm. We find that an early step in the localization pathway is recognition of localized RNAs by specific RNA-binding proteins in the nucleus. After transport into the cytoplasm, the RNP complex is remodeled and additional transport factors are recruited. These results suggest that cytoplasmic RNA localization initiates in the nucleus and that binding of specific RNA-binding proteins in the nucleus may act to target RNAs to their appropriate destinations in the cytoplasm.     

Vesicular cycling mechanisms that control auxin transport polarity

The polar transport of auxin controls many important plant growth and developmental processes. The polarity of auxin movement has long been suggested to be mediated by asymmetric distribution of auxin transport proteins, yet, until recently, little was known about the mechanisms that establish protein asymmetry in auxin-transporting cells. Now, a recent paper provides significant insight into the mechanism by which the GNOM protein controls the cycling of an auxin efflux carrier protein, PIN1, between the endosome and the plasma membrane. The dynamic movement of auxin transport proteins between internal compartments and the plasma membrane suggests mechanisms for alterations in auxin transport polarity in response to changing developmental or environmental regulation.     

Shaping development with ESCRTs

Originally identified for their involvement in endosomal sorting and multivesicular endosome (MVE) biogenesis, components of the endosomal sorting complex required for transport (ESCRT) are now known to control additional cellular functions such as receptor signalling, cytokinesis, autophagy, polarity, migration, miRNA activity and mRNA transport. The diverse cell biological functions of ESCRT proteins are translated into a pleiotropic set of developmental trajectories that reflect the wide repertoire of these evolutionarily conserved proteins.     

Microtubule-dependent mRNA transport in the model microorganism Ustilago maydis

Microtubule-dependent trafficking is essential in moving mRNAs over long distances. This transport mechanism regulates important cellular events such as determining polarity and local protein secretion. Key examples are developmental and neuronal processes studied in Drosophila melanogaster, Xenopus laevis as well as in mammalian cells. A simple eukaryotic system to uncover basic mechanisms was missing. Fungal models are generally well suited for this purpose, since transgenic strains can be generated easily by homologous recombination allowing in vivo studies at native expression levels. Substantial advances in understanding Ustilago maydis showed that this fungus fulfils important criteria to serve as model for microtubule-dependent mRNA trafficking. Here, we summarize progress focusing on target mRNAs, RNA localization elements, RNA-binding proteins, mRNPs, molecular motors and microtubule organization. This serves as the basis to discuss the novel mechanism of mRNP hitchhiking on endosomes as well as an unexpected link to unconventional secretion with its implications for applied sciences.     

Role for RNA-binding proteins implicated in pathogenic development of Ustilago maydis

Ustilago maydis causes smut disease on corn. Successful infection depends on a number of morphological transitions, such as pheromone-dependent formation of conjugation tubes and the switch to filamentous dikaryotic growth, as well as different types of mycelial structures during growth within the host plant. In order to address the involvement of RNA-binding proteins during this developmental program, we identified 27 open reading frames from the genome sequence encoding potential RNA-binding proteins. They exhibit similarities to RNA-binding proteins with Pumilio homology domains (PUM), the K homology domain (KHD), the double-stranded RNA binding motif (DSRM), and the RNA recognition motif (RRM). For 18 of these genes, we generated replacement mutants in compatible haploid strains. Through analysis of growth behavior, morphology, cyclic AMP response, mating, and pathogenicity, we identified three candidates with aberrant phenotypes. Loss of Khd1, a K homology protein containing three KHDs, resulted in a cold-sensitive growth phenotype. Deletion of khd4 encoding a protein with five KHDs led to abnormal cell morphology, reduced mating, and virulence. rrm4Delta strains were affected in filamentous growth and pathogenicity. Rrm4 is an RRM protein with a so far unique domain organization consisting of three N-terminal RRMs as well as a domain found in the C terminus of poly(A)-binding proteins. These results indicate a role for RNA-binding proteins in regulation of morphology as well as in pathogenic development in U. maydis.     

Multiple maternal proteins coordinate to restrict the translation of C. elegans nanos-2 to primordial germ cells

Although germ cell formation has been relatively well understood in worms and insects, how germ cell-specific developmental programs are initiated is not clear. In Caenorhabditis elegans, translational activation of maternal nos-2 mRNA is the earliest known molecular event specific to the germline founder cell P(4). Cis-elements in nos-2 3'UTR have been shown to mediate translational control; however, the trans-acting proteins are not known. Here, we provide evidence that four maternal RNA-binding proteins, OMA-1, OMA-2, MEX-3 and SPN-4, bind nos-2 3'UTR to suppress its translation, and POS-1, another maternal RNA-binding protein, relieves this suppression in P(4). The POS-1: SPN-4 ratio in P(4) increases significantly over its precursor, P(3); and POS-1 competes with SPN-4 for binding to nos-2 RNA in vitro. We propose temporal changes in the relative concentrations of POS-1 and SPN-4, through their effect on the translational status of maternal mRNAs such as nos-2, initiate germ cell-specific developmental programs in C. elegans.     

Directional Transport Is Mediated by a Dynein-Dependent Step in an RNA Localization Pathway

Cytoplasmic RNA localization is a key biological strategy for establishing polarity in a variety of organisms and cell types. However, the mechanisms that control directionality during asymmetric RNA transport are not yet clear. To gain insight into this crucial process, we have analyzed the molecular machinery directing polarized transport of RNA to the vegetal cortex in Xenopus oocytes. Using a novel approach to measure directionality of mRNA transport in live oocytes, we observe discrete domains of unidirectional and bidirectional transport that are required for vegetal RNA transport. While kinesin-1 appears to promote bidirectional transport along a microtubule array with mixed polarity, dynein acts first to direct unidirectional transport of RNA towards the vegetal cortex. Thus, vegetal RNA transport occurs through a multistep pathway with a dynein-dependent directional cue. This provides a new framework for understanding the mechanistic basis of cell and developmental polarity.