Cysteine protease that cleaves specifically after arginine or lysine residues. Does not cleave caspase-specific substrates. Plays a positive regulatory role in biotic and abiotic stress-induced programmed cell death. Activated by Ca(2+) which induces self-processing and accelerates the rate of the enzyme activity, but has no effect on Km. Expressed in roots, cotyledons, leaves, cauline leaves, pollen and embryos. The two subunits are derived from the precursor sequence by an autocatalytic mechanism. No visible phenotype when grown under normal conditions, but reduced sensitivity to cell death induced by the mycotoxin fumonisin B1, methyl viologen (oxidative stress) and infection with an avirulent strain of P.syringae DC3000. Plants overexpressing MCA4 are more sensitive to the mycotoxin fumonisin B1 and methyl viologen (oxidative stress) and exhibited accelerated cell-death progression. Belongs to the peptidase C14B family.