ORIGINAL PAPER A. Heyl á J. Muth á G. Santandrea á T. O'Connell A. Serna á R.D. Thompson A transcript encoding a nucleic acid-binding protein speci®cally expressed in maize seeds Received: 16 February 2001 /Accepted: 12 July 2001 / Published online: 24 August 2001 Ó Springer-Verlag 2001 Abstract A cDNA clone has been obtained for a low- abundance, seed-speci®c mRNA that encodes a poly- peptide which de®nes a novel family of plant proteins with some similarities to the DnaJ class of molecular chaperones. The MEM1 Maize Endosperm Motif binding protein) protein is capable of binding to the endosperm motif and activating transcription in the yeast one-hybrid system. Recombinant MEM1 was shown to bind in vitro to nucleic acids, with a preference for RNA over DNA. MEM1 is capable of forming homodimers, a property that is dependent on a domain close to the C-terminus of the protein. The protein is expressed in mid- to late-term endosperm cells. Sub- cellular fractionation and size fractionation under non-denaturing conditions indicate that the protein is present in the cytosol of endosperm cells. Possible roles of MEM1 in endosperm and protein body development are discussed. Keywords DNA á RNA á RNA-binding á DnaJ á Seed Introduction The major storage proteins of maize endosperm, the zeins, are deposited during the maturation phase of endospermdevelopment,whichcommences12 daysafter pollination DAP). Zeins accumulate in the lumen of the rough endoplasmic reticulum RER), and coalesce to form membrane-bound protein bodies Heidecker and Messing1986;LopesandLarkins1993;Cloreetal.1996). The mechanism by which protein bodies are assem- bled has not been fully elucidated Herman and Larkins 1999). Based on an analysis of mutants aected in zein deposition, however, the involvement of hsp70-type chaperonins or BIPs has been demonstrated Fontes et al. 1991; Marocco et al. 1991). An investigation of protein body structure using antibodies speci®c for dif- ferent zein types has shown that deposition takes place in an ordered sequence, beginning with the accumulation of the b- and c-zeins, which form a globular framework. Into this structure the d- and the a-zeins are then inte- grated, and ®nally extrude the b- and c-zeins onto the surface of the protein body Lending et al. 1988, Lending and Larkins 1989; Esen and Stetler 1992; Galili et al. 1993). The constitution of cereal protein bodies is also in¯uenced by mRNA targeting. Thus, mRNAs for rice prolamin and glutelin mRNAs are directed to separate regions of the endoplasmic reticulum, the PB-ER and C-ER, respectively, and this sorting process results in the formation of distinct types of protein bodies containing the two protein types Li et al. 1993). mRNA localisation has been widely described as a mechanism for specifying the site of synthesis of par- ticular gene products within a cell Ferrandon et al. 1994; Elisha et al. 1995). mRNA localisation involves interactions with components of the cytoskeleton Nasmyth and Jansen 1997; Hazelrigg 1998). Both microtubules and actin micro®laments may be involved in this process, depending on the cell type involved. The best characterised examples involve mRNAs whose asymmetric localisation aects a step in dierentiation, Mol Genet Genomics 2001) 266: 180±189 DOI 10.1007/s004380100561 Communicated by J. Schell A. Heyl 1 á J. Muth 2 á G. Santandrea á T. O'Connell 3 A. Serna 4 á R.D. Thompson &) MPI fuÈr ZuÈchtungsforschung, Carl-von-Linne Weg 10, 50829 KoÈln, Germany E-mail: thompson@mpiz-koeln.mpg.de Fax: +49-221-5062-413 Present addresses: 1 Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, CT 06520, USA 2 Fraunhofer Institut fuÈr Umweltchemie und O È kotoxikologie, Abt. Molekulare Biotechnologie, Auf dem Aberg 1, 57392 Schmallenberg, Germany 3 Sloning Biotechnology, Steinbergstr. 30, 85250 AltomuÈnster, Germany 4 Sainsbury Laboratory, John Innes Centre, Norwich Research Park, Colney Lane, Norwich NR4 7UH, UK