The path of RNA through nuclear pores: apparent entry from the sides into specialized pores

The path that RNA takes through nuclear pores was mapped using two high-resolution techniques. Unexpectedly, no RNA in HL60 cells was detected by immunogold labelling in the central axis of the pore complex on its way to the transporter at the nuclear membrane; instead, it was distributed around the sides, apparently entering just before the membrane. In rat liver nuclei, poly(A)(+) RNA, hnRNPs A1 and C, mrnp 41, ASF, and a phosphorylated subset of SR proteins were also distributed like mRNA, as were various transport factors and their cargoes (NTF2, Ran, RCC1, karyopherin (beta), Rch1, transportin (alpha), m(2,2,7)-trimethylG). Many pores were associated with particular transport factors/cargoes to the exclusion of others; some were associated with poly(A)(+) RNA or phosphorylated SR proteins (but not NTF2), others with NTF2 (but not poly(A)(+) RNA or the SR proteins). Electron spectroscopic imaging confirmed these results. Some pores contained phosphorus-rich RNA apparently entering from the sides; others lacked any phosphorus, and were surrounded by a ribosome-free zone in the cytoplasm. The results also suggest that pores have different functional zones where SR proteins are dephosphorylated, and where hnRNP C is removed from messages.