To gain further insight into the mode of action of S-locus receptor kinase (SRK), a receptor-like kinase involved in the self-incompatibility response in Brassica, different recombinant SRK proteins have been expressed in a membranous environment using the insect cell͞baculovirus system. Recombinant SRK proteins exhibited properties close to those of the endogenous stigmatic SRK protein and were found to autophosphorylate on serine and threonine residues in insect cell microsomes. Autophosphorylation was constitutive because it did not require the presence of pollen or stigma extracts in the phosphorylation buffer. Phosphorylation was shown to occur in trans, suggesting the existence of constitutive homooligomers of membrane-anchored recombinant SRK. To investigate the physiological relevance of these results, we have examined the oligomeric status of SRK in planta in cross-linking experiments and by velocity sedimentation on sucrose gradients. Our data strongly suggest that SRK is associated both with other SRK molecules and other stigma proteins in nonpollinated flowers. These findings may have important implications for our understanding of self-pollen signaling. baculovirus ͉ oligomerization ͉ sporophytic self-incompatibility I n the Brassica family, the cell-cell interaction that leads to the rejection of self-pollen at the stigmatic surface [the selfincompatibility (SI) reaction] is controlled genetically by the multiallelic S-locus (1). When the pollen parent and the stigma share common S haplotypes, pollen germination or pollen tube growth is inhibited, thereby preventing self-fertilization. Several genes have been localized at the S locus but only two, the S-locus glycoprotein (SLG) and the S-locus receptor kinase (SRK) genes, exhibit the polymorphic nature expected for genes involved in the SI response (2, 3). SLG encodes a secreted glycoprotein that accumulates in the cell wall of papillae (4), whereas SRK encodes a plasmalemma-anchored glycoprotein (5, 6). SRK is structurally analogous to animal receptor kinases and belongs to the plant receptor-like kinase (RLK) family (7). Although its membrane topology has not been experimentally defined, DNA sequence analysis predicts that SRK consists of three domains: an extracellular glycosylated N-terminal domain (the S-domain) that shares extensive homology with SLG, a membrane-spanning domain, and a cytoplasmic domain (3). The cytoplasmic domain has been shown to have serine͞threonine kinase activity when expressed in bacteria (8). In the S 3 haplotype, SRK has been shown to encode, in addition to the integral membrane SRK protein, a soluble truncated form corresponding to the S-domain of SRK, the eSRK protein (9).Although there is some controversy as to whether SLG is required for the SI response (10, 11), there is strong evidence that SRK is necessary. For example, SRK was shown to be mutated in two Brassica lines that exhibited a self-compatible phenotype (12, 13). Moreover, Stahl and coworkers (14) recently have shown that the SI phenotype was altered i...