MESSENGER-RIBONUCLEOPROTEIN COMPLEXES CONTAINING HUMAN ELAV PROTEINS - INTERACTIONS WITH CYTOSKELETON AND TRANSLATIONAL APPARATUS

Mammalian ELAV proteins bind to polyadenylated messenger RNAs and have specificity for AU-rich sequences, Preferred binding sites in vitro i nclude the AUUUA pentamer and related sequences present in the 3' untr anslated regions of many growth regulatory mRNAs, Human ELAV (hELAV) p roteins have been implicated in post-transcriptional regulation of gen e expression by their effects on the stability and translatability of growth regulatory mRNAs. We have examined the intracellular localizati on of ELAV proteins in neurons and in tumor cells of neuronal origin u sing indirect immunofluorescence, confocal microscopy and biochemical separation, Mammalian neuronal ELAV proteins are found predominantly i n the cytoplasm of cells in mRNP complexes termed alpha complexes whic h, when associated with polysomes, form large and high density beta co mplexes, as assayed by glycerol and accudenz gradients, respectively, Puromycin, cytochalasin or EDTA treatments disrupt beta complexes caus ing the release of alpha complexes, which then appear, by confocal mic roscopy, as large hELAV mRNP granules associated with microtubules, As sociation of partially purified hELAV mRNP alpha complexes with microt ubules was confirmed by in vitro reconstitution assays, Furthermore, c olchicine treatment of cells suggested that association of hELAV mRNP alpha complexes with microtubules is also necessary for the formation of beta complexes. Our data suggest a model in which a subset of mRNAs is associated with microtubules as ELAV mRNP particles (alpha complex es) which, in turn, associate with polysomes to form a translational a pparatus (beta complex) that is, through polysomes, associated with th e microfilament cytoskeletal network, hELAV proteins in these mRNP gra nules may affect post-transcriptional regulation of gene expression vi a the intracellular transport, localization and/or translation of grow th regulatory mRNAs.