Isolation and characterization of two beta-tubulin cDNA clones from rice

Evaluating the microtubule cytoskeleton and its interacting proteins in monocots by mining the rice genome

BACKGROUND

Microtubules (MTs) are assembled by heterodimers of alpha- and beta-tubulins, which provide tracks for directional transport and frameworks for the spindle apparatus and the phragmoplast. MT nucleation and dynamics are regulated by components such as the gamma-tubulin complex which are conserved among eukaryotes, and other components which are unique to plants. Following remarkable progress made in the model plant Arabidopsis thaliana toward revealing key components regulating MT activities, the completed rice (Oryza sativa) genome has prompted a survey of the MT cytoskeleton in this important crop as a model for monocots.

SCOPE

The rice genome contains three alpha-tubulin genes, eight beta-tubulin genes and a single gamma-tubulin gene. A functional gamma-tubulin ring complex is expected to form in rice as genes encoding all components of the complex are present. Among proteins that interact with MTs, compared with A. thaliana, rice has more genes encoding some members such as the MAP65/Ase1p/PRC1 family, but fewer for the motor kinesins, the end-binding protein EB1 and the mitotic kinase Aurora. Although most known MT-interacting factors have apparent orthologues in rice, no orthologues of arabidopsis RIC1 and MAP18 have been identified in rice. Among all proteins surveyed here, only a few have had their functions characterized by genetic means in rice. Elucidating functions of proteins of the rice MT cytoskeleton, aided by recent technical advances made in this model monocot, will greatly advance our knowledge of how monocots employ their MTs to regulate their growth and form.

Proteome analysis of rice uppermost internodes at the milky stage

Uppermost internodes, which connect the part between the ear and lower stem, form an important pathway transporting mineral nutrition from roots and photosynthates from leaves (especially the flag leaf) to the ear. The milky stage is the first stage of seed ripening. The uppermost internodes of rice at the milky stage are critical for seed quality and yield. Total soluble proteins of the uppermost internodes of rice (Oryza sativa L. ssp. indica) at the milky stage were analyzed using proteomic methods. Using 2-DE, 762 reproducible protein spots were detected. Among them, 132 abundant proteins were analyzed using MALDI-TOF-MS. Searching in the National Center for Biotechnology Information database, we could identify 98 proteins, which represent 80 gene products. These proteins belong to 11 functional groups with energy production-associated proteins in the first place. The large accumulation of proteins involved in metabolism, signaling, and stress resistance indicated that the uppermost internodes of rice have a high physiological and stress-resistant activity. In addition, our results will also enrich the database of the rice proteome.

Expression analyses of beta-tubulin isotype genes in rice

Microtubules play important roles in many cellular processes, such as cell division and cell elongation in plants. beta-tubulins, which are the basic components of microtubules, are encoded by multigene family in eukaryotes and their nucleotide sequences are highly conserved in protein coding regions. A homology search within the rice expressed sequence tag database identified at least eight beta-tubulin (OsTUB) isotypes including three novel OsTUB genes. Northern analysis using specific probes to 3'-UTR of OsTUB isotypes showed differential and tissue-specific expression. Seven out of eight OsTUB genes dominantly expressed in leaf sheath, while OsTUB8 was preferentially expressed in anther including mature pollens. The existence of anther-specific beta-tubulin suggests its unique role in the formation of microtubules during the anther and pollen development or pollen tube growth. Furthermore, transcripts of OsTUB5, 6 and 7 genes were significantly enhanced by gibberellin but all eight OsTUB genes were repressed by abscisic acid. Our results imply that OsTUB genes are differentially regulated by developmental and hormonal signals and different OsTUB isotypes might play special role in the growth and development of specific organs in rice.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Developmental expression and regulation by light of two closely related beta-tubulin genes in Lupinus albus

We present here the characterization of a Lupinus albus beta-tubulin gene, TubB2, which is closely related to TubB1. Both TubB1 and TubB2 transcripts are present in the embryonic axis of lupin dry seeds. The patterns of developmental expression of TubB1 and TubB2 beta-tubulin genes are strongly correlated. Both genes are expressed at higher levels in hypocotyls of 7-day-old etiolated plants compared to hypocotyls from plants grown under light/dark cycles. When etiolated plants are exposed to continuous white light, differential changes in the steady state levels of the TubB1 and TubB2 mRNAs from hypocotyls are observed. These changes are accompanied by an inhibition of hypocotyl extension.