BIMA(APC3), A COMPONENT OF THE ASPERGILLUS ANAPHASE PROMOTING COMPLEXCYCLOSOME, IS REQUIRED FOR A G(2) 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 G(2) which is post activation of CDC2, Here we s how that this G(2) arrest is due to loss of nimA function and that it is dependent on BIMA(APC3), component of the anaphase promoting comple x/cyclosome (APC/C). Whereas nimA single mutants arrested in G(2) with decondensed chromatin and interphase microtubule arrays, nimA, bimA(A PC3) double mutants arrested growth with condensed chromatin and aster -like microtubule arrays. nimA, bimA(APC3) double mutants entered mito sis with kinetics similar to bimA(APC3) Single mutants and wild-type c ells, indicating a checkpoint-like role for BIMA(APC3) in G(2), Even c ells which had been depleted for NIMA protein and which contained insi gnificant levels of NIMA kinase activity entered mitosis on inactivati on of bimA(APC3). BIMA(APC3) was present in a >25S complex containing BIMEAPC1, and bimA(APC3) mutants were sensitive to elevated CYCLIN B e xpression, consistent with BIMA(APC3) being a component of the APC/C, Inactivation of bimA(APC3) had little affect on the steady state level s of the B-type cyclin, NIMECyclin B, Our results indicate that BIMA(A PC3), and most likely the APC/C itself, is activated in GZ in nimA mut ants. We propose that APC/C activation is part of a novel, late G(2) c heckpoint, which responds to a defective process or structure in nimA mutants, and which prevents inappropriate entry into mitosis.