SAP130 and CSN1 interact and regulate male gametogenesis in Arabidopsis thaliana

COP9 signalosome (CSN) is a nuclear complex composed of eight distinct subunits that governs vast developmental processes in Arabidopsis thaliana (L.) Heynh. The null alleles of csn mutants display pleiotropic phenotypes that result in seedling lethality. To date, several partially complemented transgenic plants, expressing the particular CSN subunit in its corresponding null mutant allele, were utilized to bypass seedling lethality and investigate CSN regulation at later stages of development. One such transgenic plant corresponding to CSN1 subunit, fus6/CSN1-3-4, accumulates wild-type level of CSN1 and displays normal plant architecture at vegetative stage. Here we show through histological analyses that fus6/CSN1-3-4 plants display impairment of pollen development at the bicellular stage. This defect is identical to that observed in RNAi plants of SAP130, encoding a subunit of the multiprotein splicing factor SF3b. We further dissected the previously reported interaction between CSN1 and SAP130, to reveal that approximately 100 amino-acid residues located at the N-terminal end of CSN1 (CSN1NN) were essential for this interaction. In silico structure modeling demonstrated that CSN1NN could swing out towards SAP130 to dock onto its Helical Insertion protruding from the structure. These results support our model that CSN1 embeds itself within CSN protein complex through its C-terminal half and reaches out to targets through its N-terminal portion of the protein. Taken together, this is the first report to document the identical loss-of-function phenotypes of CSN1 and SAP130 during male gametogenesis. Thus, we propose that SAP130 and CSN1 coordinately regulate development of male reproductive organs.