An Arabidopsis F-box protein regulates tapetum degeneration and pollen maturation during anther development.

The Arabidopsis anther has a bilateral symmetry with four lobes, each consisting of four distinct layers of somatic cells from the outer to inner side: epidermis, endothecium, middle layer and tapetum. The tapetum is a layer of cells comprising the inner surface of the pollen wall. It plays an important role in anther development by providing enzymes, materials and nutrients required for pollen maturation. Genes and molecular mechanisms underlying tapetum formation and pollen wall biosynthesis have been studied in Arabidopsis. However, tapetum degeneration and anther dehiscence have not been well characterized at the molecular level. Here, we report that an Arabidopsis gene, designated reduced male fertility (RMF), regulates degeneration of tapetum and middle layer during anther development. The Arabidopsis dominant mutant rmf-1D overexpressing the RMF gene exhibited pleiotropic phenotypes, including dwarfed growth with small, dark-green leaves and low male fertility. Tapetum development and subsequent degeneration were impaired in the mutant. Accordingly, pollen maturation was disturbed, reducing the male fertility. In contrast, tapetum degeneration was somewhat accelerated in the RMF RNAi plants. The RMF gene was expressed predominantly in the anther, particularly in the pollen grains. Notably, the RMF protein contains an F-box motif and is localized to the nucleus. It physically interacts with the Arabidopsis-Skp1-like1 protein via the F-box motif. These observations indicate that the RMF gene encodes an F-box protein functioning in tapetum degeneration during anther development.

Molecular dissection of the APC/C inhibitor Rca1 shows a novel F-box-dependent function.

Rca1 (regulator of Cyclin A)/Emi (early mitotic inhibitor) proteins are essential inhibitors of the anaphase-promoting complex/cyclosome (APC/C). In Drosophila, Rca1 is required during G2 to prevent premature cyclin degradation by the Fizzy-related (Fzr)-dependent APC/C activity. Here, we present a structure and function analysis of Rca1 showing that a carboxy-terminal fragment is sufficient for APC/C inhibition. Rca1/Emi proteins contain a conserved F-box and interact with components of the Skp-Cullin-F-box (SCF) complex. So far, no function has been ascribed to this domain. We find that the F-box of Rca1 is dispensable for APC/C-Fzr inhibition during G2. Nevertheless, we show that Rca1 has an additional function at the G1-S transition, which requires the F-box. Overexpression of Rca1 accelerates the G1-S transition in an F-box-dependent manner. Conversely, S-phase entry is delayed in cells in which endogenous Rca1 is replaced by a transgene lacking the F-box. We propose that Rca1 acts as an F-box protein in an as yet uncharacterized SCF complex, which promotes S-phase entry.

F-box-like domains are present in most poxvirus ankyrin repeat proteins.

Vertebrate poxviruses encode numerous proteins with the ankyrin (ANK) repeat, protein-protein interaction motif but little is known about the role(s) of this large family of poxvirus proteins. We report here that the vast majority of poxvirus ANK repeat proteins share a general molecular architecture that includes a conserved amino acid motif at the carboxyl terminus. This motif is most like the F-box seen in a range of cellular proteins. From 80-100% of the ANK repeat proteins of any one poxvirus have an F-box-like domain and we observed only one poxvirus protein with an F-box-like domain but lacking ANK repeats. The proteins of only one genus of vertebrate poxviruses lack F-box-like domains and this genus does not encode ANK repeat proteins. Many F-box proteins are recognition subunits of ubiquitin ligase complexes in which the F-box binds to core elements of the complex and protein-protein interaction domains in the remainder of the protein bind the substrate protein. These observations suggest a general model of the function of the poxvirus ANK-F-box proteins. We propose that the F-box-like domains in these proteins interact with cellular ubiquitin ligase complexes and thereby direct the ubiquitination of proteins bound to the ANK repeats. The large number of different poxviral ANK-F-box proteins suggests a wide range of cellular proteins might be subjected to ubiquitin-mediated degradation, thereby modulating diverse cellular responses to viral infection.