The S locus receptor kinase gene encodes a soluble glycoprotein corresponding to the SRK extracellular domain in Brassica oleracea

Abstract

Self-incompatibility in Brassica is controlled by the S locus which contains at least two genes. SLG encodes a secreted S locus glycoprotein whilst SRK encodes a putative S locus receptor kinase which consists of three domains: an extracellular domain sharing extensive sequence identity with SLG, transmembrane region, and a cytoplasmic domain exhibiting a serine/threonine protein kinase activity. Here, the existence of truncated forms of the SRK protein corresponding to the extracellular domain of the putative receptor is reported. These proteins were detected by an antibody which recognizes the N-terminus of SRK3 and, in an F2 progeny segregating for the S3 haplotype, were only expressed in plants possessing the S3 haplotype. The truncated SRK proteins were expressed specifically in stigmas but, unlike the membrane-spanning SRK3 protein, were soluble and occurred as four different glycoforms sharing the same amino acid backbone as shown by deglycosylation experiments. Several SRK3 transcripts that may code for these truncated SRK3 proteins have been identified by RACE PCR, stigma cDNA library screening and RNA blot analysis. These transcripts are apparently generated by a combination of alternative splicing and the use of alternative polyadenylation signals. The existence of truncated forms of the S locus receptor kinase highlights some similarities between plant and animal receptor kinases. In animals, soluble extracellular domains of receptors have been described and, in some cases, have been shown to play a role in the modulation of signal transduction. By analogy, the soluble, truncated SRK proteins may play a similar role in the self-incompatibility response.

Cloning and Sequencing of cDNAs for Hypothetical Genes from Chromosome 2 of Arabidopsis,

“…Multiple polyadenylation sites have been found previously in different transcripts in plants (Giranton et al, 1995;Golovkin and Reddy, 1996;Hartung and Puchta, 2000). From our results, six of 16 cDNA sequences of hypothetical genes display two or more polyadenylation sites (Fig.…”

“…In spinach (Spinacia oleracea), there are two cDNA clones encoding stromal and thylakoid-bound ascorbate peroxidase isoenzymes (Ishikawa et al, 1996), which are produced by alternative splicing of two 3Ј-terminal exons (Ishikawa et al, 1997). In cauliflower (Brassica oleracea), a truncated SRK protein is specifically expressed in stigmata and translated from one of several transcripts, which are generated by a combination of alternative splicing and the use of alternative polyadenylation signals (Giranton et al, 1995). In maize (Zea mays), ZEMa gene encodes a MADS box-type transcription factor, for which transcripts are present in almost all maize tissues, but specific differentially spliced forms accumulate preferentially in maturing endosperm and leaf (Montag et al, 1995).…”

Self-Incompatibility in the Genus Arabidopsis: Characterization of the S Locus in the Outcrossing A. lyrata and Its Autogamous Relative A. thaliana

“…However, in most Brassica strains analyzed to date, the steady state amount of the 1.6-kb transcripts exceeds that of the 2.8-kb transcripts by as much as two orders of magnitude. The Brassica 1.6-kb transcripts have been shown to consist predominantly of transcripts derived from the SLG gene and to a lesser extent of SRK transcripts that contain the S domain-encoding exon and terminate within the first intron of the gene (Stein et al, 1991;Giranton et al, 1995). In A. lyrata, the entire 1.6-kb transcript population must be derived from the SRK gene, based on of the absence of an SLG-like gene in the Sa and Sb haplotypes we analyzed.…”

“…The modest Self-Incompatibility in the Genus Arabidopsis 639 amounts of 1.6-kb transcripts that are detected by SRK S domain probes in A. lyrata stigmas likely represent truncated SRK transcripts that correspond to the S domain of the gene. These short transcripts are produced by SRK genes (Stein et al, 1991;Giranton et al, 1995) and SRK-like genes of Arabidopsis (Tobias and Nasrallah, 1996). They are predicted to encode a secreted soluble form of the SRK ectodomain and have been shown to produce such a protein for at least one Brassica SRK allele (Giranton et al, 1995).…”

Self-Incompatibility in the Genus Arabidopsis: Characterization of the S Locus in the Outcrossing A. lyrata and Its Autogamous Relative A. thaliana

“…These short transcripts are produced by SRK genes (Stein et al, 1991;Giranton et al, 1995) and SRK-like genes of Arabidopsis (Tobias and Nasrallah, 1996). They are predicted to encode a secreted soluble form of the SRK ectodomain and have been shown to produce such a protein for at least one Brassica SRK allele (Giranton et al, 1995). The function of this class of protein is not known.…”