| map03013 |
RNA transport |
RNA transport from the nucleus to the cytoplasm is fundamental for gene expression. The different RNA species that are produced in the nucleus are exported through the nuclear pore complexes (NPCs) via mobile export receptors. The majority of RNAs, such as tRNAs, rRNAs, and U snRNAs, are transported by specific export receptors, which belong to the karyopherin-beta family proteins. A feature of karyopherins is their regulation by the small GTPase Ran. However, general mRNA export is mechanistically different. Nuclear export of mRNAs is functionally coupled to different steps in gene expression processes, such as transcription, splicing, 3'-end formation and even translation. |
| map03008 |
Ribosome biogenesis in eukaryotes |
Ribosomes are the cellular factories responsible for making proteins. In eukaryotes, ribosome biogenesis involves the production and correct assembly of four rRNAs and about 80 ribosomal proteins. It requires hundreds of factors not present in the mature particle. In the absence of these proteins, ribosome biogenesis is stalled and cell growth is terminated even under optimal growth conditions. The primary pre-rRNA transcript is assembled into the 90S pre-ribosome, which contains both 40S and 60S assembly factors. Within this complex, the pre-rRNA is cleaved. pre-60S ribosomes are subjected to several sequential processing steps in the nucleoplasm involving numerous assembly intermediates before it is exported to the cytoplasm and matured into the 60S ribosomal subunit. The pre-40S ribosome is matured to the small ribosomal subunit in the cytoplasm by cleavage. |
