Involved in the maturation of the plastid protein translocation channel. Required for the biogenesis of plastid internal membranes. May also function as a thylakoidal processing peptidase. Cleavage of hydrophobic, N-terminal signal or leader sequences from secreted and periplasmic proteins. May be located at the envelope membranes in premature chloroplast and at the thylakoidal membrane in mature chloroplasts. Plants are albino and seedling lethal. Belongs to the peptidase S26 family.