Inhibition of the apolipoprotein B mRNA editing enzyme-complex by hnRNP C1 protein and 40S hnRNP complexes.
The apolipoprotein (apo) B mRNA can be modified by a posttranscriptional base change from cytidine to uridine at nucleotide position 6666. This editing of apo B mRNA is mediated by a specific enzyme-complex of which only the catalytic subunit APOBEC-1 (apo B mRNA editing enzyme component 1) has been cloned and extensively characterized. In this study, two-hybrid selection in yeast identified hnRNP C1 protein to interact with APOBEC-1. Recombinant hnRNP C1 protein inhibited partially purified apo B mRNA editing activity from rat small intestine and bound specifically to apo B sense RNA around the editing site. The inhibition of apo B mRNA editing by hnRNP C1 protein was not due to masking of the RNA substrate as the mutant protein M104 spanning the RNA-binding domain of hnRNP C1 protein bound strongly to the apo B RNA, but did not inhibit the editing reaction. The apo B mRNA editing enzyme-complex of rat liver nuclear extracts sedimented in sucrose density gradients around 22-27S, but did not contain hnRNP C1 protein that was found exclusively within 40S hnRNP complexes. The removal of 40S hnRNP complexes increased the activity of the 22-27S editing enzyme-complex. Adding back 40S hnRNP complexes with hnRNP C1 protein resulted in an inhibition of the 22-27S apo B mRNA editing enzyme-complex, while addition of 18S fractions had no effect. In conclusion, hnRNP C1 protein identified by two-hybrid selection in yeast is a potent inhibitor of the apo B mRNA editing enzyme-complex. The abundant hnRNP C1 protein, which is contiguously deposited on nascent pre-mRNA during transcription and is involved in spliceosome assembly and mRNA splicing, is a likely regulator of the editing of apo B mRNA which restricts the activity of APOBEC-1 to limited and specific editing events.
Greeve
J
J
Medizinische Klinik, Universitäts-Krankenhaus Eppendorf, Hamburg, Germany.
Lellek
H
H
Rautenberg
P
P
Greten
H
H
D000071479:APOBEC-1 Deaminase; D000818:Animals; D001483:Base Sequence; D002467:Cell Nucleus / Q000378:metabolism; D003564:Cytidine Deaminase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D017931:DNA Primers; D005982:Glutathione Transferase / Q000235:genetics; D034482:Heterogeneous-Nuclear Ribonucleoprotein Group C; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D007422:Intestines / Q000166:cytology / Q000201:enzymology; D008099:Liver / Q000378:metabolism; D011485:Protein Binding; D017393:RNA Editing; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D012261:Ribonucleoproteins / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics; D000098830:AICDA (Activation-Induced Cytidine Deaminase)
C439788:C1 HNRNP; D017931:DNA Primers; D034482:Heterogeneous-Nuclear Ribonucleoprotein Group C; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D011993:Recombinant Fusion Proteins; D012261:Ribonucleoproteins; D005982:Glutathione Transferase; D000098830:AICDA (Activation-Induced Cytidine Deaminase); D000071479:APOBEC-1 Deaminase; C000605091:Apobec1 protein, rat; D003564:Cytidine Deaminase