EVIDENCE FOR A CYTOSKELETON-ASSOCIATED BINDING-SITE INVOLVED IN PROLAMINE MESSENGER-RNA LOCALIZATION TO THE PROTEIN BODIES IN RICE ENDOSPERM TISSUE

Previous studies have demonstrated that the mRNAs encoding the prolami ne and glutelin storage proteins are localized to morphologically dist inct membranes of the endoplasmic reticulum (ER) complex in developing rice (Oryza sativa L.) endosperm cells. To gain insight about this mR NA localization process, we investigated the association of prolamine polysomes on the ER that delimit the prolamine protein bodies (PBs). T he bulk of the prolamine polysomes were resistant to extraction by 1% Triton X-100 either alone or together with puromycin, which suggests t hat these translation complexes are anchored to the PB surface through a second binding site in addition to the well-characterized ribosome- binding site of the ER-localized protein translocation complex. Suppre ssion of translation initiation shows that these polysomes are bound t hrough the mRNA, as shown by the simultaneous increase in the amounts of ribosome-free prolamine mRNAs and decrease in prolamine polysome co ntent associated with the membrane-stripped PB fraction. The prolamine polysome-binding activity is likely to be associated with the cytoske leton, based on the association of actin and tubulin with the prolamin e polysomes and PBs after sucrose-density centrifugation.