SERK1, like BAK1, is a member of the LRRII subfamily and shares 80% amino acid sequence identity with BAK1. Immunoprecipitation of cyan fluorescent protein-tagged SERK1 from transgenic Arabidopsis plants, followed by LC/MS analysis of the immunoprecipitate, revealed that both BRI1 and BAK1 co-immunoprecipitated with SERK1 (Karlova et al. 2006). The direct in vivo physical association of these proteins was further confirmed by FLIM and FRET analysis in protoplasts. Genetic evidence for a role of SERK1 in BR signaling was demonstrated by crossing the serk1-1 mutant allele that results in a kinase inactive SERK1 protein, with a weak allele of BRI1, bri1-119. The serk1-1/bri1-119 double mutant had shorter petioles, reduced rosette size, and shorter inflorescences compared to bri1-119. This suggests a role for SERK1 in BR signaling by modulating BRI1 action in a manner similar to, although not quite as strong, as BAK1. To determine if SERK1, BAK1, and BRI1 all associated together at the same time, as opposed to multiple forms of SERK1 complexed to either BRI1 or BAK1 in the immunoprecipitate, blue native electrophoresis was employed to demonstrate a core complex of SERK1, BAK1, and BRI1 of approximately 350 kDa (Karlova et al. 2006). Three other LRRII subfamily members share high sequence similarity to BAK1 and SERK1, and it would be of interest to determine whether any of these can also interact with BRI1. Such variable multimeric complexes could work to fine-tune BRI1 function in different growth conditions.
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