Arabidopsis katanin binds microtubules using a multimeric microtubule-binding domain.

Katanin is a heterodimeric protein that mediates ATP-dependent destabilization of microtubules in animal cells. In plants, the catalytic subunit of Arabidopsis thaliana katanin (AtKSS, Arabidopsis thaliana Katanin Small Subunit) has been identified and its microtubule-severing activity has been demonstrated in vitro. In vivo, plant katanin plays a role in the organization of cortical microtubules, but the way it achieves this function is unknown. To go further in our understanding of the mechanisms by which katanin severs microtubules, we analyzed the functional domains of Arabidopsis katanin. We characterized the microtubule-binding domain of katanin both in vitro and in vivo. It corresponds to a poorly conserved sequence between plant and animal katanins that is located in the N-terminus of the protein. This domain interacts with cortical microtubules in vivo and has a low affinity for microtubules in vitro. We also observed that katanin microtubule-binding domain oligomerizes into trimers. These results show that, besides being involved in the interaction of katanin with microtubules, the microtubule-binding domain may also participate in the oligomerization of katanin. At the structural level, we observed that AtKSS forms ring-shaped oligomers.     

Two microtubule-associated proteins of Arabidopsis MAP65s promote antiparallel microtubule bundling

The Arabidopsis MAP65s are a protein family with similarity to the microtubule-associated proteins PRC1/Ase1p that accumulate in the spindle midzone during late anaphase in mammals and yeast, respectively. Here we investigate the molecular and functional properties of AtMAP65-5 and improve our understanding of AtMAP65-1 properties. We demonstrate that, in vitro, both proteins promote the formation of a planar network of antiparallel microtubules. In vivo, we show that AtMAP65-5 selectively binds the preprophase band and the prophase spindle microtubule during prophase, whereas AtMAP65-1-GFP selectively binds the preprophase band but does not accumulate at the prophase spindle microtubules that coexists within the same cell. At later stages of mitosis, AtMAP65-1 and AtMAP65-5 differentially label the late spindle and phragmoplast. We present evidence for a mode of action for both proteins that involves the binding of monomeric units to microtubules that “zipper up” antiparallel arranged microtubules through the homodimerization of the N-terminal halves when adjacent microtubules encounter.     

Cellular isolation, culture and characterization of the marrow sac cells in human tubular bone

The goal of this study is to characterize the epithelioid-like human marrow sac cells that separate the myeloid and osteoblast populations in situ and to determine if they express osteoblast cytoplasmic markers. Tubular segments of femoral diaphyseal bone were obtained from healthy young (4-8 yr) male and female patients undergoing femoral shortening surgeries. The interface between bone and marrow was examined by scanning (SEM) and transmission electron microscopy (TEM). The marrow sac cells were isolated and cultured in a-MEM medium with and without dexamethasone, glycerophosphate, and ascorbic acid [DGPA]. Alkaline phosphatase (ALP), bone morphogenetic protein-2 (BMP-2) and osteocalcin were evaluated. In the SEM, the marrow sac presented a distinctive pattern of large overlapping cells. TEM studies showed that marrow sac was one or two cells thick, which were attenuated with elongated nuclei, few cellular organelles, and appeared to display intercellular gap junctions. In culture, the marrow sac cells stained positively for ALP and BMP-2, and their expression was enhanced two- to three- fold when the cells were grown in DGPA. DGPA did not enhance osteocalcin expression. The cells of the human marrow sac reside proximate to endosteal osteoblasts and express osteoblastic markers. It is possible that these stromal cells constitute an osteoprogenitor pool from which replacement osteoblasts are recruited, and that they are involved in normal bone formation and in bone diseases (e.g., osteoporosis and osteopenia).     

Cryptococcus spp. DNA extraction from environmental samples

The genus Cryptococcus encompasses 38 species, but only 3 are associated with disease in humans and animals, Cryptococcus laurentii, Cryptococcus albidus and Cryptococcus neoformans. The last one is the most frequently reported. The disease is acquired by the inhalation of infectious propagules present in the environment. The habitat has been established using extraction techniques with buffer supplemented with antibiotics and plating in selective media. The aim of this work was to evaluate several DNA extraction techniques for Cryptocococus spp. from environmental samples. The control isolates were C. neoformans, C. albidus, C. laurentii and Paracoccidiodes brasiliensis. We also used vermiculita and soil samples contaminated with different yeast concentrations (10 to 10(6) cells/g) and samples naturally contaminated with C. neoformans. DNA was extracted with physical and chemical methods and with a commercial kit, and the DNA was purified with agarose blocks and silica columns. For the PCR amplification we used the CN4-CN5 primers, which are specific for C. neoformans. Only the commercial kit allowed DNA extraction and amplification from contaminated soil samples up to a concentration of 10 cells/g and from one sample naturally colonized. With this work we extracted and amplified DNA from Cryptococcus spp. from environmental samples with appropriate PCR specificity, it will be a tool to establish the ecological areas of C. neoformans in our country.     

Interactions of tobacco microtubule-associated protein MAP65-1b with microtubules

Tobacco microtubule associated protein (MAP65) (NtMAP65s) constitute a family of microtubule-associated proteins with apparent molecular weight around 65 kDa that collectively induce microtubule bundling and promote microtubule assembly in vitro. They are associated with most of the tobacco microtubule arrays in situ. Recently, three NtMAP65s belonging to the NtMAP65-1 subfamily have been cloned. Here we investigated in vitro the biochemical properties of one member of this family, the tobacco NtMAP65-1b. We demonstrated that recombinant NtMAP65-1b is a microtubule-binding and a microtubule-bundling protein. NtMAP65-1b has no effect on microtubule polymerization rate and binds microtubules with an estimated equilibrium constant of dissociation (K(d)) of 0.57 micro m. Binding of NtMAP65-1b to microtubules occurs through the carboxy-terminus of tubulin, as NtMAP65-1b was no longer able to bind subtilisin-digested tubulin. In vitro, NtMAP65-1b stabilizes microtubules against depolymerization induced by cold, but not against katanin-induced destabilization. The biological implications of these results are discussed.     

Modulation of Starch Digestion for Slow Glucose Release through "Toggling" of Activities of Mucosal α-Glucosidases

Starch digestion involves the breakdown by α-amylase to small linear and branched malto-oligosaccharides, which are in turn hydrolyzed to glucose by the mucosal α-glucosidases, maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI). MGAM and SI are anchored to the small intestinal brush-border epithelial cells, and each contains a catalytic N- and C-terminal subunit. All four subunits have α-1,4-exohydrolytic glucosidase activity, and the SI N-terminal subunit has an additional exo-debranching activity on the α-1,6-linkage. Inhibition of α-amylase and/or α-glucosidases is a strategy for treatment of type 2 diabetes. We illustrate here the concept of "toggling": differential inhibition of subunits to examine more refined control of glucogenesis of the α-amylolyzed starch malto-oligosaccharides with the aim of slow glucose delivery. Recombinant MGAM and SI subunits were individually assayed with α-amylolyzed waxy corn starch, consisting mainly of maltose, maltotriose, and branched α-limit dextrins, as substrate in the presence of four different inhibitors: acarbose and three sulfonium ion compounds. The IC(50) values show that the four α-glucosidase subunits could be differentially inhibited. The results support the prospect of controlling starch digestion rates to induce slow glucose release through the toggling of activities of the mucosal α-glucosidases by selective enzyme inhibition. This approach could also be used to probe associated metabolic diseases.