New twist to nuclear import: When two travel together

Ribosomes are the nanomachines that synthesize all cellular proteins from mRNA templates. In eukaryotes, ribosomes, which are composed of ribosomal proteins and rRNA, are mainly assembled in the nucleus. Thus, ribosomal proteins require a nuclear transport step from their place of synthesis in the cytoplasm to their site of assembly in the nucleus. Recognition of import substrates is mediated by different types of nuclear localization signals, which are either directly recognized by import receptors or recruited to these via adaptor proteins. The novel transport adaptor Syo1 (Symportin), which is dedicated to the synchronous import of two functionally related ribosomal proteins, has recently been described. In this review, we highlight and discuss these findings in the context of our current knowledge of ribosome assembly and nucleocytoplasmic transport. We propose that nuclear co-import of functionally and topologically linked cargo could be a widespread strategy to streamline assembly of macromolecular complexes in the nucleus.