Little is known about the molecular mechanism of assembly of ribosomal protein complexes. A leucine zipper motif mediates dimerization of a number of different proteins, including a class of DNA-binding proteins (Landschulz et al., Science 240:1759-1764, 1988). The leucine zipper motif is a sequence of leucine residues spaced every seventh amino acid residue along an alpha-helix. As a result, all the leucine residues of the motif are placed along one side of the alpha-helix. According to the zipper model, the leucine side chains extending from the leucine repeat are able to interdigitate with leucine side chains of a second polypeptide which contains the same motif. Therefore, the hydrophobic surfaces of two leucine repeating sequences of the protein molecules might interact to form homo or heterodimers (Landschulz et al., Science 240:1759-1764, 1988; Kouzarides and Ziff, Nature 336:646-651, 1988; Zerial et al., EMBO J. 8:805-813, 1989). The leucine zipper model for protein dimerization is strongly supported by experimental data which show that replacement of leucine at any position of the motif destabilises dimer formation as well as the DNA-binding potential of the protein (Kouzarides and Ziff, Nature 336:646-651, 1988). Leucine zipper motifs have been reported in photosystem I reaction centre polypeptides of higher plants (Webber and Malkin, FEBS Lett. 264:1-4, 1990), glucose-transporter glycoproteins (Buckland and Wild, Nature 338:547, 1989) and voltage-gated Ca2+ channels (White and Weber, Nature 340:103-104, 1989). It appears reasonable to assume that the leucine zipper motif is a common mechanism for mediating protein dimer formation in a wide range of different systems when specific interactions are required.
We report here the presence of an unusual leucine motif formed by leucine repeats (-Leu-X10-Leu-) in mitochondrial ribosomal protein S13 of maize and other plant species (Figure 1). This new amino acid motif -L-X10-L-X10-M-X10-L-X10-L- is located at aa positions 37-81. The characteristic feature of the given motif in comparison with the known leucine motifs in proteins of different origin is the disruption of leucine regularity of the motif by incorporation in its composition of another hydrophobic amino acid with a long side chain, methionine, in the position 59 of the amino acid sequence.
Z.m. msyisgarslpdeqvriastkmdgigpkkaiqlryr L gisgnikihe L tkyqidqieq Miaqdhvvhwe L
T.ae. msyisgarslpdeqvriastkmdgigpkkaiqiryr L gisgnikmne L tkyqidqieq Miaqdhvvhwe L
E.s. mxyixgarslpdeqvriastkmdgigpkkaiqiryr L gisxnikmne L tkyqidqieq Miaqdhvvhwe L
M.p. msyilgtnlnsnkqvkialtrifgigpkkaiqvcdq L gisdtikvkk L tkyqfdqilk Mmsqnylvdse L
N.t. mlyisgarlvgdeqvriastkidgigpkkaiqvryr L gisgnikike L tkyqidqieq Migqdhvvhwe L
Z.m. krgeradier L isisryrgirhqdgsplrgqrthtnartarkqiwkgnerrlpkeqatd
T.ae. krgeradier L isisryrgirhqdgsplrgqrthtnartarkqiwk - 116
E.s. krgeradier L isisryrgirhqdgsplrgqrthtnartarkxxxk - 116
M.p. krviqrdikr L isigcyrgfrhnaglplrgqrthgnaktcrklryvsirs - 120
N.t. krgeradier L isiscyrgirhqdgsplrgqrthtnartcrklirk - 116
Figure 1. Leucine motif in amino acid sequence of the rps13 of maize and other plant species. One-letter code is used. Z.m., Zea mays; T.ae., Triticum aestivum; E.s., Elymus sibiricus; M.p., Marchantia polymorpha; N.t., Nicotiana tabacum. All amino acid sequences are from database EMBL 1993 Year October (release 36.0). Leucine and methionine residues forming motif are shown by capital letters.
Protein secondary structure prediction by the method of Garnier (Garnier et al., J. Mol Biol. 120:97-120, 1978) showed that the region including the motif has an alpha-helical conformation (data not shown). Since leucine (including one methionine) 11 amino acids apart will appear on the same side of an alpha-helical wheel, this suggests that this new hydrophobic amino acid motif is perhaps involved in ribosomal protein-protein interaction through helix-helix association by analogy with the leucine zipper motif.
We assume also that this highly conserved amino acid motif may provide
specific recognition sites during assembly of the ribosomal supramolecular
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