Oocyte-Specific Expression of Mouse MEX3C652AA in the Ovary and Its Potential Role in Regulating Maternal Fos mRNA.

Currently, the human MEX3C gene is known to encode an RNA-binding protein of 659 amino acid residues. Here we show that the MEX3C gene has alternative splicing forms giving rise to multiple MEX3C variants, and some cells express MEX3C transcripts coding for short MEX3C isoforms but not transcripts for MEX3C(659AA). MEX3C(659AA) functions as an adaptor protein for Exportin 1 (XPO1)-mediated nuclear export since it increases the cytoplasmic distribution of poly(A)(+) RNA and since addition of the nuclear export signal (NES) sequence to a short MEX3C isoform MEX3C(464AA) confers similar cytoplasmic poly(A)+ RNA accumulation activity as MEX3C(659AA). FOS mRNA is a potential MEX3C target mRNA. One mechanism by which MEX3C(659AA) could regulate FOS mRNA is by promoting its nuclear export. Overexpressing MEX3C(659)AA significantly increased FOS mRNA expression, whereas mutating the NES of MEX3C(659AA) and treating cells with leptomycin B to inhibit XPO1-mediated nuclear export attenuated FOS upregulation. FOS mRNA is unstable in somatic cells but less so in oocytes; how it is stabilized in the oocytes is unknown. Transcripts for the mouse counterpart of human MEX3C(659)AA (MEX3C(652AA)) are specifically expressed in developing oocytes in the ovary, although total Mex3c transcripts are expressed in both granulosa cells and oocytes. The specific expression of this long MEX3C isoform in oocytes and its ability to enhance FOS mRNA nuclear export and stability all suggest that MEX3C(659AA) is an RNA-binding protein that preserves maternal FOS mRNA in oocytes.