Head assembly. Head assembly has been subdivided in five steps although most steps are not very well understood in mechanistic terms. The tail is assembled independently. The C protease, the scaffolding protein Nu3, and the portal protein (B) form an ill-defined initiator structure. Protein E joins this complex but the chaperonins GroES and GroEL are required for that step. Within the prohead C and E are processed to form covalently joined X1 and X2 proteins although this is controversial (see text). Proteins Nu1, A, and FI are required for DNA packaging. Protein D joins and stabilizes the capsid as a structural protein. FII and W are connecting the head to the tail that joins once the head is completed. Modified after  and .