Primary structural features of the self-incompatibility protein in solanaceae

Abstract

We present a sequence comparison of 12 Sallele-associated proteins from three solanaceous species with gametophytic self-incompatibility: Nicotiana alata, Petunia inflata, and Solanum chacoense. The allelic variants of the S-protein exhibit a very high degree of sequence diversity consistent with their function as recognition molecules. We identify 41 perfectly conserved residues, 18 of which are also conserved in two fungal ribonucleases, RNase T2 and RNase Rh. The residues conserved in both the S-proteins and the ribonucleases include two histidines essential for catalysis, four cysteines involved in disulfide bridges, and hydrophobic residues probably involved in the core structure of the proteins. This conservation between the two ribonucleases and the 12 divergent S-proteins confirms the previously recognized similarity between 3 more closely related N. alata S-proteins and these ribonucleases, and argues strongly for the functional importance of the ribonuclease activity of the S-protein in self-incompatibility. We also identify the 19 most variable residues, which are the prime candidates for the S-allele-specificity determinant. Twelve of these nineteen residues are clustered in two regions of hypervariability, designated HVa and HVb, which are also the most prominent hydrophilic regions of the S-protein. We suggest that these two regions might form parts of the putative pollen recognition site. Identification of these structural features forms a foundation for the study of the molecular basis of selfrecognition and the biochemical mechanism of inhibition of self-pollen tube growth.

Structural and Transcriptional Analysis of the Self-Incompatibility Locus of Almond: Identification of a Pollen-Expressed F-Box Gene with Haplotype-Specific Polymorphism

“…Amino acid sequence alignment showed that SFB contained two quite variable regions, as did S-RNase (Ioerger et al, 1991;Ushijima et al, 1998). The hypervariable region of S-RNase is exposed on the surface of the folded protein molecule and plays a pivotal role in the recognition of self-pollen (Matton et al, 1997(Matton et al, , 1999Ida et al, 2001;Matsuura et al, 2001).…”

“…Sequence identities among SFBs were 68.4 to 76.4% (Table 2), which were comparable to those of almond S-RNases (54.2 to 76.2%) (Ushijima et al, 1998;Tamura et al, 2000). Although variable sites are dispersed in the primary structure of the SFB, two regions at the C terminus are quite variable, as they are in the hypervariable region of the S-RNase (Ioerger et al, 1991;Ushijima et al, 1998).…”

Functional gametophytic self-incompatibility in a peripheral population of Solanum peruvianum (Solanaceae)

“…We used the First Strand cDNA Synthesis Kit (EMD Biosciences, Inc., Madison, WI, USA) to synthesize cDNAs from the S-RNase encoding mRNAs. A portion of the S-RNase gene (spanning conserved regions C2-C5, see Ioerger et al, 1991) was amplified from cDNA using degenerate primers PR1 and PR3 from Richman et al, 1995. PCRs were performed in 50-ml reactions including 5 ml template, 10 mM Tris-HCl buffer, 50 mM KCl, 1.5 mM MgCl 2 , 0.2 mM dNTPs, 100 ng of each primer, 5 mg BSA and 2 units of Taq polymerase.…”

Rapid recent radiation of S-RNase lineages in Witheringia solanacea (Solanaceae)

“…Invitrogen's cDNA cycle kit was used to generate cDNA. Degenerate primers PR1 and PR3 (Richman et al, 1995), based on conserved regions C2 and C5 (Ioerger et al, 1991), were used to amplify approximately 490 base pairs of the SRNase gene. Thermal cycler conditions were modified for use with a thermocycler with a heated lid (941C, 15 s; 451C, 60 s; 721C, 60 s).…”

“…The alignment was checked by eye to ensure that conserved motifs and cysteine residues were properly aligned (Ioerger et al, 1991). Small insertions/deletions combined with highly variable sequences created problems aligning the ultimate seven to nine amino acids; for analysis, the terminal 4-5 amino acids of the sequences were truncated following the conserved cysteine at amino-acid position 119/120 (position 160 of Ioerger et al, 1991). PHYLIP v3.573c (Felsenstein, 1995) was used to construct a neighbor-joining tree.…”