SmMAK16, the Schistosoma mansoni homologue of MAK16 from yeast, targets protein transport to the nucleolus

The SmMAK16 gene from Schistosoma mansoni was cloned by chance when an adul t worm cDNA library was probed with antiserum to affinity-purified S. manso ni GSH S-transferases. SmMAK16 encodes a hydrophilic protein of 259 amino a cids with a molecular mass of 31 kDa. The protein shares 43% sequence ident ity and 66% similarity to the nuclear protein MAK16 of Saccharomyces cerevi siae that has been implicated both in cell cycle progression and biogenesis of 60S ribosomal subunits. Both proteins display a similar degree of seque nce similar to the hypothetical protein CeMAK16 from Caenorhabditis elegans . These proteins share a number of apparent protein motifs, including two n uclear localization signals (NLS), multiple sites for phosphorylation by pr otein kinase CK2 and four conserved cysteine residues that resemble a zinc binding domain. SmMAK16 mRNA is more highly expressed in adult female worm than males. Recombinant SmMAK16 was phosphorylated by human protein kinase CK2. When chimeric constructs containing SmMAK16 fused the green fluorescen t protein (GFP) were transiently transfected into COS-7s cells, the reporte r was localized not in nuclei, but exclusively in nucleoli. The yeast and n ematode homologues were likewise able to direct nucleolar accumulation of t he fluorescent reporter. The high degree of sequence conservation together with the ability to direct nucleolar protein transport supports the hypothe sis that MAK16 proteins play a key role in the biogenesis of 60S subunits. (C) 2000 Elsevier Science B.V. All rights reserved.