Localization of rRNA transcribed spacer domains in the nucleolinus and maternal procentrosomes of surf clam (Spisula) oocytes

Abstract

The nucleolinus is a nuclear subcompartment long ago posited to play a role in cell division. In a recent study using surf clam oocytes, cytoplasmic foci containing a nucleolinar protein were shown to later recruit γ-tubulin, identifying them as centrosomal precursors.¹ We now demonstrate the presence of structural RNAs from the nucleolinus in these procentrosomes. They include the well-known but poorly understood rRNA-transcribed spacer regions. In situ hybridization revealed a specific and dynamic association of these structural RNAs with the cell division apparatus that extends through the early stages of meiosis. In addition to their bearing on the debate over the nature of centrosome- and spindle-associated RNAs, the observations also suggest that rRNA spacer regions are not simply waste products to be discarded immediately, but may be functional byproducts that play a role in formation of the cell division apparatus.

Keywords: :

• centrosome
• nucleolus
• nucleolinus
• rRNA transcribed spacers
• non-coding RNA

Acknowledgments

This work was supported by a grant from the NIH (GM088503) to M.C.A.

Notes

a In this report, centrosomes are defined as the two γ-tubulin-containing foci that form in the cytoplasm between 4 and 5 min post-fertilization and take up positions at the spindle poles during the course of meiosis. Procentrosomes are defined as cytoplasmic nucleolinin-containing foci that appear within the first 2 min post-fertilization and become γ-tubulin-positive shortly thereafter.

b Note that spindle integrity and α-tubulin antigenicity in these hybridized samples is severely compromised due to the required proteinase-K treatment and stringent hybridization conditions, including 2 d at 60°C. The spindle images shown in Figure 4 are therefore of low quality, but still reveal the position and extent of the spindle, and are consistent with observations using γ-tubulin antibody to reveal centrosomes.¹