Post-transcriptional maturation of the S receptor kinase of Brassica correlates with co-expression of the S-locus glycoprotein in the stigmas of two Brassica strains and in transgenic tobacco plants.
The S-locus-encoded S receptor kinase (SRK) is an intrinsic plasma membrane protein that is viewed as the primary stigma determinant of specificity in the self-incompatibility response of Brassica spp. We analyzed two self-compatible mutant strains that express low levels of the S-locus glycoprotein (SLG), a cell wall-localized protein also encoded at the S locus that is coordinately expressed with SRK. We found that mutant stigmas synthesized wild-type levels of SRK transcripts but failed to produce SRK protein at any of the developmental stages analyzed. Furthermore, SRK was shown to form aberrant high-molecular mass aggregates when expressed alone in transgenic tobacco (Nicotiana tabacum) plants. This aggregation was prevented in tobacco plants that co-expressed SRK and SLG, but not in tobacco plants that co-expressed SRK and SLR1, an SLG-related secreted protein not encoded at the S locus. In analyses of protein extracts under reducing and non-reducing conditions, evidence of intermolecular association was obtained only for SLG, a fraction of which formed disulfide-linked oligomers and was membrane associated. The data indicate that, at least in plants carrying the S haplotypes we analyzed, SRK is an inherently unstable protein and that SLG facilitates its accumulation to physiologically relevant levels in Brassica stigmas.
Dixit
R
R
Department of Plant Biology, Cornell University, Ithaca, New York 14853, USA.
Nasrallah
M E
ME
Nasrallah
J B
JB
D001937:Brassica / Q000235:genetics* / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D018506:Gene Expression Regulation, Plant; D006023:Glycoproteins / Q000235:genetics* / Q000378:metabolism; D015151:Immunoblotting; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism; D018514:Plant Structures / Q000235:genetics / Q000378:metabolism; D030821:Plants, Genetically Modified; D010947:Plants, Toxic; D020033:Protein Isoforms; D011494:Protein Kinases / Q000235:genetics* / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism*; D014026:Nicotiana / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic
D006023:Glycoproteins; D010940:Plant Proteins; D020033:Protein Isoforms; D012333:RNA, Messenger; C064156:self-incompatibility locus specific glycoprotein, Brassica; D011494:Protein Kinases; C079832:S-receptor kinase