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Plant Cell Advance Online Publication
Published on October 6, 2006; 10.1105/tpc.106.041061

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Received January 16, 2006
Returned for revision July 19, 2006
Accepted September 11, 2006

Identification and Characterization of Components of a Putative Petunia S-Locus F-Box-Containing E3 Ligase Complex Involved in S-RNase-Based Self-Incompatibility

Zhihua Hua 1 and Teh-hui Kao 2*

1 Intercollege Graduate Degree Program in Plant Biology, Pennsylvania State University, University Park, Pennsylvania 16802
2 Intercollege Graduate Degree Program in Plant Biology, Pennsylvania State University, University Park, Pennsylvania 16802; Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park, Pennsylvania 16802

* To whom correspondence should be addressed. E-mail: txk3{at}psu.edu.

Petunia inflata S-locus F-box (Pi SLF) is thought to function as a typical F-box protein in ubiquitin-mediated protein degradation and, along with Skp1, Cullin-1, and Rbx1, could compose an SCF complex mediating the degradation of nonself S-RNase but not self S-RNase. We isolated three P. inflata Skp1s (Pi SK1, -2, and -3), two Cullin-1s (Pi CUL1-C and -G), and an Rbx1 (Pi RBX1) cDNAs and found that Pi CUL1-G did not interact with Pi RBX1 and that none of the three Pi SKs interacted with Pi SLF2. We also isolated a RING-HC protein, S-RNase Binding Protein1 (Pi SBP1), almost identical to Petunia hybrida SBP1, which interacts with Pi SLFs, S-RNases, Pi CUL1-G, and an E2 ubiquitin-conjugating enzyme, suggesting that Pi CUL1-G, SBP1, and SLF may be components of a novel E3 ligase complex, with Pi SBP1 playing the roles of Skp1 and Rbx1. S-RNases interact more with nonself Pi SLFs than with self Pi SLFs, and Pi SLFs also interact more with nonself S-RNases than with self S-RNases. Bacterially expressed S1-, S2-, and S3-RNases are degraded by the 26S proteasomal pathway in a cell-free system, albeit not in an S-allele-specific manner. Native glycosylated S3-RNase is not degraded to any significant extent; however, deglycosylated S3-RNase is degraded as efficiently as the bacterially expressed S-RNases. Finally, S-RNases are ubiquitinated in pollen tube extracts, but whether this is mediated by the Pi SLF-containing E3 complex is unknown.






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