The pathway of bacteriophage T4 head assembly begins with the formation of a prohead bound to the bacterial cell membrane which is later converted to the mature, DNA-containing head. During maturation, all but one of the prohead proteins are proteolytically processed by a phage-coded protease which is formed by autocatalytic cleavage of the product of gene 21 (gp21). Protease gp21 has been tentatively located in the center of the prohead core. Belongs to the peptidase U9 family.