Import and Export of UsnRNPs
The biogenesis of spliceosomal UsnRNPs in higher eukaryotes requires a cytoplasmic maturation step. After transcription and initial processing in the nucleus, the snRNAs U1, U2, U4 and U5 are exported to the cytoplasm in an m7G‑Cap dependent manner utilising the PHAX/CRM1 dependent pathway. In the cytoplasm, these UsnRNAs specifically associate with seven Sm‑proteins that form a doughnut‑shaped UsnRNP core structure. This assembly process, which is mediated by the SMN complex, is a prerequisite for the hypermethylation of the m7G‑cap to the 2,2,7‑trimethylguanosine (m3G)‑cap. Snurportin1 specifically recognises this m3G‑cap and in concert with other import factors facilitates the import of core UsnRNPs into the nucleus. The status quo of knowledge with respect to the two transport steps in UsnRNP biogenesis will be discussed.