MESSENGER-RNA BINDING-PROTEIN MRNP-41 LOCALIZES TO BOTH NUCLEUS AND CYTOPLASM

We have identified and molecularly characterized a human protein with a M-r of 40,880 Da, After UV irradiation of HeLa cells, this protein w as cross-linked to poly(A)-containing mRNA and was therefore designate d mrnp 41 (for mRNA binding protein of 41 kDa), Cell fractionation and immunoblotting showed mrnp 41 in both the cytoplasm and the nucleus a nd particularly in the nuclear envelope. Immunofluorescence microscopy localized mrnp 41 to distinct foci in the nucleoplasm, to the nuclear rim, and to meshwork-like structures throughout the cytoplasm, The cy toplasmic meshwork staining was disrupted by prior treatment of cells with the actin filament-or microtubule-disrupting drugs cytochalasin o r nocodazole, respectively, suggesting association of mrnp 41 with the cytoskeleton, Double immunofluorescence with antibodies against mrnp 41 and the cytoplasmic poly(A) binding protein showed colocalization t o the cytoplasmic meshwork, Immunogold electronmicroscopy confirmed mr np 41's cytoplasmic and nucleoplasmic localization and revealed a stri king labeling of nuclear pore complexes, Together these data suggest t hat mrnp 41 may function in nuclear export of mRNPs and/or in cytoplas mic transport on, or attachment to, the cytoskeleton, Consistent with a role of mrnp 41 in nuclear export are previous reports that mutation s in homologs of mrnp 41 in Schizosaccharomyces pombe, designated Rae1 p, or in Saccharomyces cerevisiae, designated Gle2p, result in mRNA ac cumulation in the nucleus although it is presently not known whether t hese homologs are mRNA binding proteins as well.