The timely and organized form of programmed cell death known as apoptosis has vital functions in the development and pathologies of complex metazoan life forms. Apoptosis is regulated by conserved signaling pathways that activate caspase proteases leading to target protein cleavage and cell death. Morphological features typical of apoptosis have been recognized in single cell organisms and structural homologues of the caspase family (metacaspases) have been identified 
. Metacaspase proteases have since been characterized as apoptotic regulators in plants, fungi and protozoa supporting the concept of a widely conserved cell death pathway 
A single metacaspase, YOR197w, was identified in the genome of S. cerevisiae
and named Y
, (Yca1p also known as MCA1). Yca1p demonstrates enzymatic peptidase activity analogous to mammalian caspase activity, and is inhibited in the presence of the fluoromethyl ketone (FMK) conjugated caspase-inhibitory substrate z-VAD-FMK 
. In addition to canonical caspase substrate specificity Yca1p has secondary substrate specificity directed at arginine-lysine motifs 
. Furthermore, Yca1p-mediated apoptosis has been observed in a variety of apoptogenic situations including senescence and following insults such as hydrogen peroxide (H2
) and acetic acid 
. These findings have promoted the view of yeast as an evolutionarily distant model of metazoan apoptosis and numerous orthologous pathways have since been reported.
Despite the discovery of homologues to apoptotic proteins in yeast, there was limited consensus that yeast engaged a death program similar to that of multi-cellular metazoans. Arguments to rationalize apoptosis in yeast have included altruism in the presence of reactive oxygen species or killer toxins, a program to execute cells that fail to mate in response to pheromone and a mechanism for lateral gene transfer 
. In addition, the apoptotic pathway in yeast has been suggested to act as a mechanism to protect surrounding cells from the potentially damaging cellular constituents of dead or dying cells 
. However, the validity of these hypotheses has yet to be fully explored and may not justify the development and retention of a biochemical pathway devoted solely to cell death. Without an incontrovertible justification for an apoptotic response in yeast, it is also reasonable to conjecture that proteins such as Yca1p may have initially developed or co-evolved other vital non-death functions 
Caspase activity is an inductive signal for non-death functions such as cell cycle progression, proliferation, cell migration and differentiation in mammalian cell lines 
. Enzymatic processing of target proteins by caspase-3 at key time points promotes differentiation in mammalian precursor and progenitor cells 
. The enzymatic activity of caspases has also been associated with the proper activation and release from the mitotic checkpoint. In HeLa cells, disruption of microtubule dynamics results in caspase mediated cleavage of the mitotic checkpoint protein BubR1p, augmenting the duration of mitosis 
. Similarly, inhibition of caspase activity in human hepatoma cells causes the failure of metaphase arrest in the presence of microtubule depolymerizing nocodazole 
. Various apoptogenic insults can promote death at specific phases in the cell cycle indicating that the cell-cycle and apoptosis are intimately linked 
. Apoptosis of G2/M arrested cells is a widespread observation in the literature 
. Finally, caspase-8 is activated in EGF-stimulated primary hepatocytes and participates in mitogenic signaling as a positive regulator of G1/S transition 
. Collectively these findings suggest that a non-death role for caspases in the mammalian cell cycle exists and, depending on the system/conditions, can modulate the fidelity of major cell cycle checkpoints.
In this study we describe a non-death role for the yeast metacaspase Yca1p. DNA content analysis of Δyca1 vs. wild-type shows that Yca1p accelerates G1/S and antagonizes G2/M transitions granting Yca1p-expressing cells an increased fitness in fermentive conditions. Analysis of a Yca1p catalytic inactivation mutant revealed that enzymatic activity of Yca1p is upstream of the G1/S transition phenotype. The slow growth phenotype of the Δyca1 strain was exacerbated in the presence of the catalytic inactivation mutant resulting in an even longer procession through the S and G2/M cell cycle phases. Furthermore, genetic or chemical inhibition of Yca1p results in desensitization to nocodazole. These findings support the hypothesis that the canonical apoptotic proteins also regulate critical non-death functions in single cell organisms.