a key question concerning an activity that inhibits apc function is how it is regulated in the cell cycle and under conditions of spindle damage . the most straightforward hypothesis is that improperly attached kinetochores will catalyze the formation of a diffusible apc inhibitory form . ( 2001 ) report provocative findings that suggest this may not be the case for the bubr1-containing mcc complex . they first characterized the activity of  mitotic  mcc , isolated from cells treated with a microtubule - depolymerizing drug and therefore expected to be activated for checkpoint function . surprisingly , they subsequently found mcc with equivalent inhibitory activity and subunit composition in interphase cells , a stage in which mature kinetochores are not present and the spindle checkpoint should not be activated . their further experimentation suggests that the ability of the mcc to inhibit the apc is not specified by the state of the mcc but is in fact a regulated property of the apc . apc isolated from mitotic cells was sensitive to mcc inhibition , but interphase apc was not . ( 2001 ) also report that the apc exhibits cell cycle stage - specific changes in its susceptibility to inhibition by pure bubr1 . ( 2001 ) also show that the addition of chromosomes purified from cells arrested in mitosis could enhance inhibition of apc in vitro . this effect appeared to be mediated by an action of the mitotic chromosomes upon the apc as opposed to its activator cdc20 or its inhibitor mcc . the chromosome effect may be a result of the reported ability of kinetochores to biochemically  remember  that they were not properly attached to the spindle despite cell - free purification ( campbell et al . , 2000 ) . how might the apc 's sensitivity to mitosis - specific phosphorylation of apc subunits is well documented , and preferential association of bubr1 , cdc20 , and mad2 to phosphorylated apc has been reported ( sudakin et al . , 2001 ) . perhaps phosphorylation events required for inhibition are mediated by one of the spindle checkpoint kinases , mps1 , bub1 , or even bubr1 . another possibility is that the 95% of cellular mad2 not associated with mcc participates in apc sensitization . future studies must determine the mechanism of apc sensitization to inhibition in order to solidify this new view of checkpoint action . this new focus on bubr1 is of great interest for another reason . by virtue of its association with the kinetochore - based motor protein cenp - e , bubr1 has also been implicated in an early checkpoint step , leading to the generation of the prevent - anaphase signal . proper bipolar spindle attachment in metaphase causes sister kinetochores to be placed under tension , since they are pulled towards opposite poles while still held together by cohesive forces . an undetermined aspect of either the failure of kinetochores to attach or their failure to be placed under bipolar tension ( or both ; skoufias et al . , 2001 ) is sensed by the checkpoint mechanism . kinesin - related cenp - e is a kinetochore - based microtubule motor protein that has been implicated in kinetochore attachment to microtubules and/or tension generation . interference with its function prevents proper metaphase chromosome alignment ( yao et al . , 2000 ) . depending on the experimental system , cenp - e function was found to either be required for the checkpoint response or required to prevent a bubr1-dependent checkpoint arrest ( chan et al . , 1999 ; localization of cenp - e to kinetochores requires the actions of two checkpoint proteins , mps1 and bub1 ( abrieu et al . , 2001 ; sharp - baker and chen , 2001 ) . finally , cenp - e associates with bubr1 during m phase of the cell cycle ( chan et al . , 1998 ; yao et al . , 2000 ) . thus , in addition to its newly demonstrated role in the output of the spindle checkpoint pathway , bubr1 may act at an early step as well . although the functional nature of the bubr1cenp - e interaction is not known , it is tempting to speculate that together they monitor an aspect of kinetochore attachment or tension . besides missing the bub1r - like kinase domain , yeast cells perhaps the bubr1 kinase domain , apparently not required for apc inhibition , functions with cenp - e in kinetochore - based checkpoint signal generation .
previous studies of the spindle checkpoint suggested that its ability to prevent entry into anaphase was mediated by the inhibition of the anaphase - promoting complex ( apc ) ubiquitin ligase by mad2 . 
 two new studies challenge that view by demonstrating that another checkpoint protein , bubr1 , is a far more potent inhibitor of apc function .