The Arabidopsis anaphase promoting complex (APC) - Regulation through subunit availability in plant tissues

Sister-chromatid separation and exit from mitosis require ubiquitin-mediated proteolysis of cell cycle regulators such as cyclin B and securin. The specificity of the reaction is controlled by an ubiquitin-ligase multiprotein complex known as APC (Anaphase Promoting Complex). Comparison of the coding sequences of Arabidopsis genes with the Genbank database reveals extensive homology of the predicted ORFs with the corresponding proteins of other eukaryotes, indicating that the APC is well conserved in plants. However, different from other eukaryotes, the Arabidopsis genes have some particular characteristics, such as the presence of two copies of the CDC27 gene. Furthermore, expression analyses of the AtAPC genes disclose complex profiles that differ, depending on the tissue examined. In actively dividing cell suspensions there is a direct correspondence between the rates of proliferation and mRNA levels from the AtAPC components. On the other hand, in plant organs, dark-grown seedlings and during leaf growth, this correlation is lost and the AtAPC genes are highly expressed in tissues with low overall cell division. Moreover, expression patterns diverge between the subunit genes, raising the possibility that there could be more than one form of the APC, which would execute distinct functions during plant development. The results suggest that an important layer of regulation of APC/C in plants could operate through subunit availability in specific tissues and/or cellular compartments.