Membrane estrogen receptors: genomic actions and post transcriptional regulation

The primary cellular location of the nuclear estrogen receptor II (nER II) is the plasma membrane. A number of reports that have appeared in the recent past indicate that plasma membrane localized estrogen receptor alpha (ERalpha) also exists. Whether the membrane localized ERalpha represents the receptor that binds to the estrogen responsive element (ERE) remains to be known. The mechanisms that underlie the internalization of nER II (non-activated estrogen receptor, deglycosylated) have been identified to a certain extent. The question remains: is the primary location of the ERalpha also the plasma membrane? If that is the case, it will be a challenging task to identify the molecular events that underlie the plasma membrane-to-nucleus movement of ERalpha. The internalization mechanisms for the two 66kDa plasma membrane ERs, following hormone binding, appear to be distinct and without any overlaps. Interestingly, while the major gene regulatory role for ERalpha appears to be at the level of transcription, the nER II has its major functional role in post transcriptional mechanisms. The endoplasmic reticulum associated anchor protein-55 (ap55) that was recently reported from the author's laboratory needs a closer look. It is a high affinity estrogen binding protein that anchors the estrogen receptor activation factor (E-RAF) in an estrogen-mediated event. It will be interesting to examine whether ap55 bears any structural similarity with either ERalpha or ERbeta.