Sea urchin microtubules

The C. elegans EMAP-like protein, ELP-1 is required for touch sensation and associates with microtubules and adhesion complexes

Background

The founding member of the EMAP-like protein family is the Echinoderm Microtubule-Associated Protein (EMAP), so-named for its abundance in sea urchin, starfish, and sand dollar eggs. The EMAP-like protein family has five members in mammals (EML1 through EML5) and only one in both Drosophila (ELP-1) and C. elegans (ELP-1). Biochemical studies of sea urchin EMAP and vertebrate EMLs implicate these proteins in the regulation of microtubule stability. So far, however, the physiological function of this protein family remains unknown.

Results

We examined the expression pattern of C. elegans ELP-1 by means of transgenic gene expression in living embryos and adults, and by immunolocalization with an ELP-1-specific antibody in fixed tissues. In embryos, ELP-1 is expressed in the hypodermis. In larvae and adults, ELP-1 is expressed in the body wall, spermatheca and vulval muscles, intestine, and hypodermal seam cells. In muscle, ELP-1 is associated with adhesion complexes near the cell surface and is bound to a criss-crossing network of microtubules in the cytoplasm. ELP-1 is also expressed in a subset of mechanoreceptor neurons, including the ray neurons in the male tail, microtubule-rich touch receptor neurons, and the six ciliated IL1 neurons. This restricted localization in the nervous system implies that ELP-1 plays a role in mechanotransmission. Consistent with this idea, decreasing ELP-1 expression decreases sensitivity to gentle touch applied to the body wall.

Conclusion

These data imply that ELP-1 may play an important role during the transmission of forces and signals between the body surface and both muscle cells and touch receptor neurons.

Sea urchin vault structure, composition, and differential localization during development

Background

Vaults are intriguing ribonucleoprotein assemblies with an unknown function that are conserved among higher eukaryotes. The Pacific coast sea urchin, Strongylocentrotus purpuratus, is an invertebrate model organism that is evolutionarily closer to humans than Drosophila and C. elegans, neither of which possesses vaults. Here we compare the structures of sea urchin and mammalian vaults and analyze the subcellular distribution of vaults during sea urchin embryogenesis.

Results

The sequence of the sea urchin major vault protein (MVP) was assembled from expressed sequence tags and genome traces, and the predicted protein was found to have 64% identity and 81% similarity to rat MVP. Sea urchin MVP includes seven ~50 residue repeats in the N-terminal half of the protein and a predicted coiled coil domain in the C-terminus, as does rat MVP. A cryoelectron microscopy (cryoEM) reconstruction of isolated sea urchin vaults reveals the assembly to have a barrel-shaped external structure that is nearly identical to the rat vault structure. Analysis of the molecular composition of the sea urchin vault indicates that it contains components that may be homologs of the mammalian vault RNA component (vRNA) and protein components (VPARP and TEP1). The sea urchin vault appears to have additional protein components in the molecular weight range of 14–55 kDa that might correspond to molecular contents. Confocal experiments indicate a dramatic relocalization of MVP from the cytoplasm to the nucleus during sea urchin embryogenesis.

Conclusions

These results are suggestive of a role for the vault in delivering macromolecules to the nucleus during development.

Isolation of centrosomes from Spisula solidissima oocytes

We have described methods for the preparation of lysates and isolation of centrosomes from parthenogenetically activated oocytes of the surf clam, S. solidissima. Although oocyte availability is seasonal, between June and August as much as 2 liters of lysate can be generated by a single person. Since lysate can be stored frozen at -80 degrees C with no apparent loss in centrosome-dependent microtubule nucleation, this is a convenient system for year-round experimentation. On average, per milliliter of frozen-stored lysate, 2 or 3 x 10(6) centrosomes can be obtained at 3000- or 4000-fold purification by sucrose-density gradient centrifugation. Centrosome fractions typically contain 6.0 x 10(-12) g of protein per centrosome (Vogel et al., 1997) and 140-200 micrograms of protein is usually obtained from a single run involving six sucrose-density gradients (12 ml of lysate). One person can easily run three preparations in a day, and thus 420-600 micrograms of centrosome protein could be prepared daily. Therefore, based on the effort of one individual, as much as 20-40 mg of centrosome protein could be prepared per year. Another convenient feature of the system is that once centrosomes are isolated, they can be stored in high sucrose media at -80 degrees C for years with little or no loss in microtubule nucleation potential. Once isolated, centrosomes can be used for protein analysis, ultrastructural studies, or in functional reconstitution assays (Vogel, 1997). In addition, these preparations offer the isolation of sufficient quantities of centrosome proteins to be used as antigens for generating centrosome-specific antibodies or for obtaining protein sequence for the purpose of antibody production or the design of oligonucleotide primers for isolating cDNA fragments coding for centrosome proteins. Thus, the preparations described offer a biochemical approach for defining centrosome composition. The methods described for immunofluorescence analysis of asters assembled in lysates offer rapid and convenient preparations for screening antibodies for centrosome localization and specificity. Finally, the ability to prepare large quantities of homogeneous centrosomes should enhance ultrastructural studies since many centrosomes can be sectioned and analyzed simultaneously by EM, avoiding the problem of having to hunt through sections of single cells to find a single centrosome for analysis. In addition, colloidal gold localization studies, using antibodies and EM to pinpoint the relative location of individual proteins, could be carried out on populations of centrosomes in the same preparation simultaneously, thus drastically expanding the quantity of data gathered. In conclusion, the clam oocyte system described here offers the potential for a combined structural and biochemical approach for identification of novel centrosome proteins and elucidation of the molecular basis of microtubule nucleation, centrosome assembly, and centrosome function.

Purification of a WD repeat protein, EMAP, that promotes microtubule dynamics through an inhibition of rescue

The major microtubule-associated protein in echinoderms is a 77-kDa, WD repeat protein, called EMAP. EMAP-related proteins have been identified in sea urchins, starfish, sanddollars, and humans. We describe the purification of sea urchin EMAP and demonstrate that EMAP binding to microtubules is saturable at a molar ratio of 1 mol of EMAP to 3 mol of tubulin dimer. Unlike MAP-2, MAP-4, or tau proteins, EMAP binding to microtubules is not lost by cleavage of tubulin with subtilisin. In addition to binding to the microtubule polymer, EMAP binds to tubulin dimers in a 1:1 molar ratio. The abundance of EMAP in the egg suggests that it could function to regulate microtubule assembly. To test this hypothesis, we examined the effects of EMAP on the dynamic instability of microtubules nucleated from axoneme fragments as monitored by video-enhanced differential interference contrast microscopy. Addition of 2.2 microM EMAP to 21 microM tubulin results in a slight increase in the elongation and shortening velocities at the microtubule plus ends but not at the minus ends. Significantly, EMAP inhibits the frequency of rescue 8-fold without producing a change in the frequency of catastrophe. These results indicate that EMAP, unlike brain microtubule-associated proteins, promotes microtubule dynamics.