APC-targeted RAA1 degradation mediates the cell cycle and root development in plants

Abstract

Protein degradation by the ubiquitin-proteasome system is necessary for a normal cell cycle. As compared with knowledge of the mechanism in animals and yeast, that in plants is less known. Here we summarize research into the regulatory mechanism of protein degradation in the cell cycle in plants. Anaphase-promoting complex/cyclosome (APC), in the E3 family of enzymes, plays an important role in maintaining normal mitosis. APC activation and substrate specificity is determined by its activators, which can recognize the destruction box (D-box) in APC target proteins. Oryza sativa root architecture-associated 1 (OsRAA1) with GTP-binding activity was originally cloned from rice. Overexpression of of OsRAA1 inhibits the growth of primary roots in rice. Knockdown lines showed reduced height of seedlings because of abnormal cell division. OsRAA1 transgenic rice and fission yeast show a higher proportion of metaphase cells than that of controls, which suggests a blocked transition from metaphase to anaphase during mitosis. OsRAA1 co-localizes with spindle tubulin. It contains the D-box motif and interacts with OsRPT4 of the regulatory particle of 26S proteasome. OsRAA1 may be a cell cycle inhibitor that can be degraded by the ubiquitin-proteasome system, and its disruption is necessary for the transition from metaphase to anaphase during root growth in rice.

Figure 1

Phylogenetic tree of Oryza sativa root architecture-associated 1 (OsRAA1), Arabidopsis thaliana flowering promoting factor 1 (AtFPF1) and related small G proteins. The tree was constructed by use of CLUSTALW software and amino acid sequences.

Figure 2

Possible network of OsRAA1 regulating the development of primary roots in rice. OsRAA1 functions as an inhibitor to block the cell cycle at the transition from metaphase to anaphase during primary root development. OsRAA1 might bind to an unknown APC adaptor and be ubiquitinated by the E3 ligase APC. APC could be the specific E3 that recognizes OsRAA1 and ligates the ubiquitin chain to OsRAA1 with the help of E1 and E2. Ubiquitin-OsRAA1 interacts with the OsRPT4 subunit in the regulatory particle of 26S proteasome. Disruption of OsRAA1 is mediated by 26S proteasome and ensures the onset of anaphase in mitosis during root development. Then, the cell cycle moves into anaphase, and the primary root shows normal growth.