Purification and nucleic-acid-binding properties of a Saccharomyces cerevisiae protein involved in the control of ploidy.

Scp160p (Saccharomyces cerevisiae protein involved in the control of ploidy), a polypeptide with a molecular mass of around 160 kDa, is associated with the nuclear envelope and the endoplasmic reticulum. The most noteworthy phenotype of SCP160 deletion mutants is a decrease in viability and an increased number of chromosomes in the surviving cells [Wintersberger, U., Kühne, C. & Karwan, A. (1995) Yeast 11, 929-944]. Scp160p contains 14 KH domains, conserved motifs that have lately been identified in a variety of RNA-binding proteins. In this report, we demonstrate that the Scp160p sequence shows nearly perfect colinearity with the putative gene product of C08H9.2 from the nematode Caenorhabditis elegans as well as with the vigilins, vertebrate RNA-binding proteins with a cellular location similar to that of Scp160p. Moreover, we found that Scp160p contains a potential nuclear-export signal (NES) near its N-terminus and a potential nuclear-localization signal (NLS) between KH domains 3 and 4. To determine whether the protein is able to bind to RNA, we purified Scp160p from yeast cell extract by DNA-cellulose and anti-Scp160p affinity chromatography. In northwestern blotting experiments, the electrophoretically homogeneous protein bound to ribohomopolymers and ribosomal RNA as well as to single-stranded and double-stranded DNA. Subcellular fractionation studies revealed that the major part of Scp160p is membrane associated via ionic interactions and can be released from the membrane fraction under conditions that lead to a dissociation of ribosomes. Together, our findings suggest that Scp160p is the yeast homologue of the vigilins, and point to a role for Scp160p in nuclear RNA export or in RNA transport within the cytoplasm.