In a search for genes with enriched expression in germline stem cells, we identified the C. elegans ortholog of the nucleolar GTPase nucleostemin. Nucleostemin has an evolutionarily conserved function in controlling cell proliferation in mammals, newt, Xenopus laevis, and Saccharomyces cerevisiae (Tsai, R.Y. and McKay, R.D., 2002, Maki, N. et al., 2006, Beekman, C. et al., 2006, BaBler, J. et al., 2001). Yeast nucleostemin,
nug1, has been shown to export
pre-60S ribosomal subunits out of the nucleolus (BaBler et al., 2001). A role for mammalian nucleostemin in ribosome biogenesis has not been directly examined. We were interested in determining whether the regulation of cell proliferation by nucleostemin was directly linked to a function in ribosome synthesis. Using a rescuing translational GFP reporter construct, we have found that C. elegans nucleostemin, NST-1, is ubiquitously expressed in the nucleolus and nucleoplasm in both the soma and germ line. NST-1 does not co-localize with NOP-1/fibrillarin, an essential component of the U3 snoRNP that is directly involved in rRNA processing. This lack of co-localization is also seen with mammalian nucleostemin and suggests that
nst-1 may not participate in the transcription or early processing of rRNA (Politz et al., 2005). We generated a deletion allele of
nst-1,
vr6, that causes larval arrest due to an apparent growth defect within four hours of postembryonic development. Total RNA and RT-PCR data reveal that these arrested animals exhibit reduced levels of rRNA, which possibly accounts for this growth arrest. Loss of
nst-1 specifically in the germ line and not the soma results in sterility with severely decreased germ cell number. This is consistent with a requirement for
nst-1 in the germ line. The germline phenotype is likely due to under-proliferation, as shown by decreased anti-pH3 staining. We hypothesize that
nst-1 is necessary for exporting factors essential for ribosome biogenesis, in a manner similar to NUG1s export of
pre-60S subunits. We are currently examining whether NST-1 shuttles between the nucleolus and nucleoplasm as do mammalian and yeast nucleostemin. We are also determining whether
nst-1(
vr6) mutants exhibit aberrant localization of the
pre-60S ribosomal subunit RPL-25.2 using a translational fusion strain. It is not well known how ribosome biogenesis is coordinated with cell growth and proliferation. Thus, we hope that by elucidating how
nst-1 functions in C. elegans, we will gain a better understanding of how nucleolar proteins modulate cell proliferation.