Role of the F‐box protein exchange factor in the reproductive growth of Arabidopsis

Abstract

Cullin-Associated NEDD8 Dissociated Protein 1 (CAND1) is one of the key factors regulating the Skp1-Cul1-F-box protein (SCF) ubiquitin ligases. SCFs are enzyme complexes catalyzing the ubiquitination reaction, which tags cellular proteins with a chain of ubiquitin and designates them for subsequent degradation. In this way, SCFs can specify the half-lives of important regulatory proteins, and therefore, they play essential roles in numerous biological processes, such as hormone signaling, cell cycle regulation, immune response, and organ development. All SCFs have a core Cul1 scaffold protein, and it assembles with a member from the F-box protein family to form different SCFs with distinct substrate specificity. Akin to an exchange factor, CAND1 functions by increasing the rate at which an F-box protein dissociates from Cul1 and thus facilitating the recycling of the limited Cul1 scaffold of SCF complexes. The discovery of the CAND1 working mechanism has led to the adaptive exchange hypothesis, in which CAND1-mediated exchange enables cells to tolerate varying levels of F-box proteins and ensures the assembly of the right type of SCF at the right time for timely substrate ubiquitination. To test this hypothesis, an F-box protein was over-expressed in cultured human cells with or without the CAND1 gene, and cells lacking CAND1 became inviable upon the F-box protein over-expression while the normal cells were unaffected. This result provides the first evidence supporting the hypothesis, and it suggests that a CAND1-defective multicellular organism may fail to maintain cell types that contain a large pool of F-box proteins. Thus, we further investigated the role of CAND1 in the development of a multicellular organism. While the CAND1 knockout mice are reported to be homozygous lethal, homozygous cand1 knockout mutant Arabidopsis are viable, exhibiting impaired growth and development at various stages. Particularly, cand1 mutant Arabidopsis are almost sterile, and our studies with cell biological, physiological and bioinformatic analyses have shown that the diminished fertility was due to the death of pollen, which contains the largest pool of F-box protein transcripts among all analyzed tissues. These findings are consistent with results from studies using cultured human cells, and they provide natural evidence supporting the adaptive exchange hypothesis.