Electron tomography reveals posttranscriptional binding of pre-mRNPs to specific fibers in the nucleoplasm.
Using electron tomography, we have analyzed whether the Balbiani ring (BR) pre-mRNP particles in transit from the gene to the nuclear pore complex (NPC) are bound to any structure that could impair free diffusion through the nucleoplasm. We show that one-third of the BR particles are in contact with thin connecting fibers (CFs), which in some cases merge into large fibrogranular clusters. The CFs have a specific protein composition different from that of BR particles, as shown by immuno-EM. Moreover, we have identified hrp65 as one of the protein components of the CFs. The sequencing of hrp65 cDNA reveals similarities with hnRNP proteins and splicing factors. However, hrp65 is likely to have a different function because it does not bind to nascent pre-mRNA and is not part of the pre-mRNP itself. Taken together, our observations indicate that pre-mRNPs are not always freely diffusible in the nucleoplasm but interact with fibers of specific structure and composition, which implies that some of the posttranscriptional events that the pre-mRNPs undergo before reaching the NPC occur in a bound state.
Miralles
F
F
Department of Molecular Genome Research, Stockholm University, SE-106 91 Stockholm, Sweden.
Ofverstedt
L G
LG
Sabri
N
N
Aissouni
Y
Y
Hellman
U
U
Skoglund
U
U
Visa
N
N
D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D002467:Cell Nucleus / Q000378:metabolism / Q000648:ultrastructure*; D002683:Chironomidae; D002875:Chromosomes / Q000648:ultrastructure*; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000235:genetics; D019476:Insect Proteins; D008854:Microscopy, Electron / Q000379:methods; D008954:Models, Biological; D008961:Models, Structural; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000235:genetics / Q000302:isolation & purification*; D012322:RNA Precursors / Q000302:isolation & purification* / Q000378:metabolism; D012323:RNA Processing, Post-Transcriptional; D012333:RNA, Messenger / Q000302:isolation & purification* / Q000378:metabolism; D016601:RNA-Binding Proteins; D012261:Ribonucleoproteins / Q000302:isolation & purification*; D012469:Salivary Glands / Q000648:ultrastructure; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid
D018076:DNA, Complementary; D019476:Insect Proteins; D009687:Nuclear Proteins; D012322:RNA Precursors; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D012261:Ribonucleoproteins; C404122:hrp65 protein, Chironomus tentans