The resulting two ribosomal subunits then leave the nucleus again, to finally join into a whole ribosome out in the cytoplasm. This, of course, is a simplified description and none of the hows of each step is known. As a tool to study ribosomal export out of the nucleus, graduate student Terence Moy has devised a new assay that allows him to trace the whereabouts of the small ribosomal subunit in the cell. For comparison, this specialized region within the nucleus is labeled in b and d. After the ribosomal subunit enters the cytoplasm, wild-type yeast cells degrade the piece of RNA picked up by the assay.
Ribosomes make up a significant portion of nuclear traffic.
An average yeast cell makes 2,400 ribosomes every minute, for which its nucleus must import 3,100 ribosomal proteins and export 80 subunits per second. The first paper addresses the problem of how mRNAs in general exit the nucleus. Researchers know that in the nucleus mRNAs occur packaged in large particles that are formed as RNA-binding proteins coat the mRNA strand. By studying a new mutant strain, the scientists found that under conditions of stress, the Npl3 proteins detach from the mRNA and, remarkably, leave the nucleus on their own. We think this is a key event in mRNA export.