FUNCTIONAL AND STRUCTURAL ANALYSIS OF THE BC-1 RIBONUCLEOPROTEIN PARTICLE PRESENT IN DENDRITE OF NEURON.

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BC-1 RNA is a brain-specific small RNA transcript of identifier sequences present in the somas and dendrites of neurons. We recently reported that the RNA is complexed with a protein(s) to form a 10S ribonucleoprotein particle. We demonstrate that this10S BC-1 ribonucleoprotein particle contains a DNA-binding protein(s)(Bp-1 protein) capable of interacting with a region between split promoter sequences for RNA polymerase III within the identifier sequences. The region has short inverted repeats : aperfect octanucleotide repeat and an imperfect heptanucleotide repeat, each of which contains a GCAAG/CTTGC motif. We also demonstrate that the binding of this protein either to the array of pentamer motifs or to BC-1 RNA is mutually exclusive. The mol-ecular masses of photocross-linking adducts of Bp-1 protein to a 32P-labeled GCAAG/CTTGC motif-specific probe were estimated to be about 31 and 36 kDa, indicating that two species of Bp-1 proteins may be present in the brain.We also demonstrate an accumulation of Bp-1 protein in the nucleus of brain cells from mouse fetus and newborns that precedes the postnatal increase in BC-1 RNA.Cyt-oplasmic Bp-1 protein present in a complex with BC-1 RNA increases postnatally with a concomitant decrease in nuclear Bp-1 protein. These observations suggest that Bp-1 protein may play a role(s) in the synthesis and nuclear export of BC-1 RNA.