During the asexual blood stage, initiates the proteolytic maturation of several rhoptry proteins and thus, is required for merozoite invasion of host erythrocytes and probably the subsequent development of the ring-stage (PubMed:29074774). Cleaves rhoptry associated protein 1 RAP1 and apical sushi protein ASP during schizont maturation (PubMed:29074774). Also cleaves rhoptry protein RON3 (By similarity). In schizonts, localizes to the bulb of rhoptries. Expressed during the asexual blood stage, in schizonts and free merozoites (at protein level). Autocleaved into a p55 mature form. Normal asexual development in host erythrocytes; however, erythrocyte invasion by newly released merozoites is severely impaired (PubMed:29074774). Abnormal rhoptry morphology and impaired processing of rhoptry-associated protein RAP1 (PubMed:29074774). Belongs to the peptidase A1 family. It is unclear if PMIX is glycosylated as other members of the same enzyme family, i.e. PMI and PMII, are not.