**12. Nuclear estrogen receptor II (nERII)**

Long before naER discovery became a reality, a nuclear receptor that was distinctly different from the classical estrogen receptors had come to my notice. It was observed that when uterine nuclei from ovarectomized rats were exposed to 10nM 3H-estradiol,at 30-370C, the hormone-binding component moved out of the nuclei and reached the outer medium within a span of 5 minutes after hormonal exposure (Thampan,1985;1988). What became apparent in the subsequent studies was that the hormone was bound to a class of ribonucleoproteins (RNP) that moved out of the nuclei following exposure to estradiol. Invivo studies involving ovariectomized rats demonstrated that the RNP that moved out of the nuclei was found associated with cytoplasmic polysomes. The results gave a clear indication to the possibility that a new class of estrogen receptors existed whose primary functional role was in posttranscriptional control mechanisms like splicing, nucleocytoplasmic transport of RNP and the translation.

The subsequent studies reported by our group(Jacob et al.,2006) presented systematic observations on both naER and nERII and concluded that the latter was a transformed form of the former. The observed differences between the two proteins are being listed below(Table 1). The methods employed for purifying the two proteins were identical.

Going back to the observations related to E-RAF function (Thampan,1989), it may be recalled that the estrogen receptor function detected in close proximity to rat uterine nuclear RNA polymerases displayed both naER and nERII characteristics with nERII representing the major share of this activity. The naER existed in dimerisation with E-RAF.Later reports by Karthikeyan and Thampan (1996) showed that nERII tyrosine phosphorylated three subunits of nuclear RNA polymerase II.A re-examination of the 1989 report(Thampan,1989) will reveal that the naER/nERII interaction was not restricted to RNA polymerase II alone. There was very clear evidence to support the hypothesis that the receptor interacted with all four classes of nuclear RNA polymerases. Therefore, it may be speculated that nERII –mediated tyrosine phosphorylation involved specified subunits of all 4 categories of the enzyme.

Evolving Trends in Estrogen Receptor Biology 77

The figure on the left displays the interaction of naER-E-RAF heterodimer with genetic elements. While E-RAF recognizes the DNA, naER binds to nuclear RNA polymerase subunits. The figure on the right is a spliceosome -set -up in which the nERII –RNA complex

nERII is a RNA binding estrogen receptor. Whether naER-E-RAF heterodimer has its binding site on the target gene different from those of the ERα/ERβ mediated gene regulation or whether the action of the heterodimer is independent of the classical estrogen receptor function remains to be clarified. The binding site on the estrogen responsive target gene for E-RAF-naER heterodimer has not yet been identified while there is every likelihood to suggest that it will be different from the estrogen responsive element(ERE).A candidate site could well be AP-1 site in view of an earlier observation that c-fos and E-RAF share immunological similarity. While E-RAF binds to the gene, naER interacts with the nuclear RNA polymerases. Possibly, the naER to nERII transformation could be an event that takes place at the end of the transcription process initiated by the heterodimer. At this stage, nERII dissociates from E-RAF and binds to the RNA (rRNA/mRNA/5S rRNA/tRNA). I wish to propose here that the phosphorylated subunits of the RNA polymerases might dissociate from the core enzyme and move along with nERII during the succeeding stages of gene

is shown in association with subunits dissociated from RNA polymerase.

regulation that witness splicing, nucleocytoplasmic transport and translation.

nucleocytoplasmic movement of RNP.

**subunit.**

Sebastian and Thampan (2002 a,b)and Sebastian et al(2004) presented some fascinating observations in this context. Goat uterine nERII was found to be associated with ribonucleoproteins containing U-1 and U-2 snRNA's. Within the snRNP framework nERII interacted with three proteins with molecular masses 32kDa, 55kDa and 60kDa.While p55 and p60 were found to be RNA binding proteins, p32 was found to be involved only in protein-protein interactions with nERII. Whether this protein is the same as SC35 reported by Parnaik in the context of spliceosome assembly (Tripathi & Parnaik,2008) remains to be seen. It was interesting to observe that nERII in association with p32 and p55 formed an effective Ca++/Mg++ activated ATPase that appeared to be directly involved in the

(1) nERII (2) rRNA (3) RNA polymerase I subunit(4)40S ribosomal subunit(5)60S ribosomal subunit. Fig. 7. **A hypothetical representation for the association of nERII with 40S ribosomal** 


The factor responsible for this transformation was subsequently found to be a 61kDa protein, the naER transforming factor (Jaya & Thampan,2000).
