BIMA(APC3), component of the Aspergillus anaphase promoting complex/cyclosome, is required for a G₂ checkpoint blocking entry into mitosis in the absence of NIMA function

Temperature sensitive (ts) nimA mutants of Aspergillus nidulans arrest at a unique point in G₂ which is post activation of CDC2. Here we show that this G₂ arrest is due to loss of nimA function and that it is dependent on BIMA(APC3), a component of the anaphase promoting complex/cyclosome (APC/C). Whereas nimA single mutants arrested in G₂ with decondensed chromatin and interphase microtubule arrays, nimA, bimA(APC3) double mutants arrested growth with condensed chromatin and aster-like microtubule arrays. nimA, bimA(APC3) double mutants entered mitosis with kinetics similar to bimA(APC3) single mutants and wild-type cells, indicating a checkpoint-like role for BIMA(APC3) in G₂. Even cells which had been depleted for NIMA protein and which contained insignificant levels of NIMA kinase activity entered mitosis on inactivation of bimA(APC3). BIMA(APC3) was present in a > 25S complex containing BIME(APC1), and bimA(APC3) mutants were sensitive to elevated CYCLIN B expression, consistent with BIMA(APC3) being a component of the APC/C. Inactivation of bimA(APC3) had little affect on the steady state levels of the B-type cyclin, NIME(Cyclin B). Our results indicate that BIMA(APC3), and most likely the APC/C itself, is activated in G₂ in nimA mutants. We propose that APC/C activation is part of a novel, late G₂ checkpoint, which responds to a defective process or structure in nimA mutants, and which prevents inappropriate entry into mitosis.