F-box Proteins in Arabidopsis

In a review article, “F-box proteins in Arabidopsis”, in the November 2000 issue of Trends in Plant Science (Trends Plant Sci. 2000 Nov;5(11):454-7), Drs. Wenyan Xiao and Jyan-Chyun Jang at Ohio State University, give a detailed account  about F-box proteins in Arabidopsis and presents the current status of our knowledge in this field.

The article starts with an account of  F-box proteins  found in a number of organisms including plants where there are only a few reports. In plants, F-proteins have been identified through the study of mutants defective in specific responses.  Some of the mutants studied for this purpose are: TIR1 (transport inhibitor response) regulating auxin responses, COI1 (coronatine insensitive 1) involved in fertility and jasmonate-regulated response to pathogens and insects, UFO (unusual floral organs) required for normal patterning and developement of floral meristem.
Among the plant F-box proteins, SCFTIR1* function in auxin is the most explicit.  In loss-of-function mutants of either TIR1 or ASK1 gene, there is a reduction in auxin response; on the other hand, overexpression of TIR leads to enhanced auxin response. The results of a yeast two-hybrid assay indicate that a mutation  in the F-box abolishes the  function of  TIR1 because it fails to interact with ASK1 and ASK2.
Using conserved residues of the F-box motif and the relevant database, the authors identified 38 putative F-box proteins. The authors  have characterized these F-proteins in accordance whether they contain WD-40 repeats or leucine-rich repeats. Analysis of their structures revealed there are 25 putative F-box proteins containing LRR, two contain WD-40 repeats, and 15 contain not well characterized C-terminus motifs.
Ubiquitin-mediated proteolysis in probably conserved among eukaryotes. Most components in the pathway and subunits of the proteosome have been identified in plants. Previous studies have shown that many important biological processes in plants are controlled by proteolysis. Among these processes the following deserve special mention:  hormone response, flower development, photomorphogenesis, stress response, senescence and circidian rhythm.
In conclusion, the authors envision that other F-box proteins might be involved in cellular processes. To determine their roles, there needs to be “an optimal approach to obtain and characterize the corresponding mutants and to define the biological functions of these putative plant F-box proteins.”
*SCF= TIR1 together with ASK1 (Arabidopsis Skp-like) and AtCUL (Arabidopsis cullin-like) forms a ubiquitin ligase complex called SCF

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