Enables cysteine-type endopeptidase activity. Involved in aggrephagy and protein processing. Located in cytoplasm. Expressed in several structures, including P3.p hermaphrodite; P4.p hermaphrodite; P8.p hermaphrodite; head muscle; and somatic nervous system. Is an ortholog of human ATG4A (autophagy related 4A cysteine peptidase).