T4 mutants in gene 17 accumulate particles which contain the main head protein in the cleaved form (gp23*) arranged in an unexpanded lattice (empty small particles), together with other expanded capsids (empty large particles). The isolated empty small particles can be transformed in vitro, by lowering the ionic strength, to capsid-like structures. This structural transformaton is not coupled to chemical modification of the structural proteins of the empty small particles. In contrast to unexpanded particles that are easily dissociated, the transformed structures are as resistant to dissociation as other T-even head-related particles with expanded lattice. Furthermore, the transformed particles are able to bind in vitro hoc and soc proteins, rendering capsids indistinguishable from the normal T4 capsids both morphologically and by their stability against denaturing agents. Our results indicate that the in vitro transformation of the empty small particles might mimic important and characteristic aspects of the in vivo maturation of T4 heads, thus suggesting a possible role of the "cleaved but unexpanded" particle in the maturation pathway of the T4 shell.

Carrascosa, J. L. (1978). Head maturation pathway of bacteriophages T4 and T2. IV. In vitro transformation of T4 head-related particles produced by mutants in gene 17 to capsid-like structures. Journal of Virology, 26(2), 420–428.