 Most proteins , ribosomal subunits , and some DNAs are transported through the pore complexes in a process mediated by a family of transport factors known as karyopherins . Those karyopherins that mediate movement into the nucleus are also called importins , whereas those that mediate movement out of the nucleus are called exportins . Most karyopherins interact directly with their cargo , although some use adaptor proteins . Steroid hormones such as cortisol and aldosterone , as well as other small lipid @-@ soluble molecules involved in intercellular signaling , can diffuse through the cell membrane and into the cytoplasm , where they bind nuclear receptor proteins that are trafficked into the nucleus . There they serve as transcription factors when bound to their ligand ; in the absence of ligand , many such receptors function as histone deacetylases that repress gene expression . 
