Identification of a cytoskeleton-associated 120 kDa RNA-binding protein indeveloping rice seeds

During rice seed development, prolamine RNAs are localized to the surface o f the prolamine storage protein bodies (PBs), organelles bounded by the end oplasmic reticulum (ER). The exact mechanism by which prolamine RNAs are en riched on this ER subdomain is not known but recent evidence indicates the directed transport and targeting of prolamine RNAs to the prolamine PBs. As such a process involves RNA signal determinants and cytoskeleton-interacti ng proteins that recognize these signals, we obtained an enriched cytoskele ton-PB fraction and identified a prominent RNA-binding activity, Rp120, by RNA-binding UV-cross-linking assay. Recombinant cDNA clones of Rp120 reveal ed that the primary sequence shared considerable structural homology to the human transcriptional coactivator p100 and possessed a modular organizatio n, four nucleic acid-binding SN domains, a tudor domain and a coil-coil dom ain. Consistent with the presence of SN domains, Rp120 binds a variety of R NAs including prolamine RNA. Interaction with the latter RNA, however, was specific as binding activity was evident only to the prolamine 3' UTR and n ot to the 5' UTR or coding sequences. Rp120 is also able to interact with o ther proteins as its sedimentation behavior in sucrose density gradient sug gests an association with the cytoskeleton. The presence of a tudor domain, suggested to have a role in RNA processing or transport, together with the SN and coiled-coil domains are consistent with the view that Rp120 may be involved in RNA sorting in rice endosperm.