Transport from the Active Gene to the Nuclear Membrane Pores

How pre-mRNA is transported from the site of transcription - the active gene - to the nuclear membrane pores is not well known. According to some experimental data, the pre-mRNA-protein complex is released from the gene and then freely diffuses through the interchromatin space, eventually reaching the nuclear pores.
Export signals in proteins associated with the pre-mRNA, such as hnRNP A1 and hnRNP K, are presumably recognized by putative export receptors which triggers the attachment to, and translocation through the nuclear pore complex.
Other data shows that the pre-mRNA-protein complexes can be retained in the nucleus. Unspliced pre-mRNA is known to be retained and the hnRNP C protein contains a nuclear retention signal. It has also been reported that pre-mRNA has a resticted location in the nucleus. It is therefore possible that pre-mRNA is at least transiently attached to nuclear structures at some point along its path to the nuclear membrane pores.