Jl. Giranton et al., THE S-LOCUS RECEPTOR KINASE GENE ENCODES A SOLUBLE GLYCOPROTEIN CORRESPONDING TO THE SRK EXTRACELLULAR DOMAIN IN BRASSICA-OLERACEA, Plant journal, 8(6), 1995, pp. 827-834
Self-incompatibility in Brassica is controlled by the S locus which co
ntains 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 i
dentity with SLG, a transmembrane region, and a cytoplasmic domain exh
ibiting a serine/threonine protein kinase activity. Here, the existenc
e of truncated forms of the SRK protein corresponding to the extracell
ular domain of the putative receptor is reported. These proteins were
detected by an antibody which recognizes the N-terminus of SRK(3) and,
in an F-2 progeny segregating for the S-3 haplotype, were only expres
sed in plants possessing the S-3 haplotype. The truncated SRK proteins
were expressed specifically in stigmas but, unlike the membrane-spann
ing SRK(3) protein, were soluble and occurred as four different glycof
orms sharing the same amino acid backbone as shown by deglycosylation
experiments. Several SRK(3) transcripts that may code for these trunca
ted SRK(3) proteins have been identified by RACE PCR, stigma cDNA libr
ary screening and RNA blot analysis. These transcripts are apparently
generated by a combination of alternative splicing and the use of alte
rnative polyadenylation signals. The existence of truncated forms of t
he 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, t
he soluble, truncated SRK proteins may play a similar role in the self
-incompatibility response.