Phage lambda DNA packaging is accompanied by prohead expansion, due to structural changes in gpE, the major capsid protein. Rearrangement of the gpE lattice creates binding sites for trimers of gpD, the head stabilization protein. lambda-Like phage 21's shp gene is homologous to lambda's D gene. gpD and gpShp share 49% amino acid identity. To ask whether gpShp could stabilize the lambda head shell, we replaced lambda's D gene with shp, creating lambda shp. Unlike lambda or 21, lambda shp was strictly dependent on the presence of 10(-2) M Mg2+, and lambda shp virions were very sensitive to chelating agents. Density gradient studies indicated that the lambda gpE lattice was underpopulated with gpShp. gpD's N-terminus has been proposed to contact gpE, and we found that lambda D/shp, which produces a chimeric protein with the N-terminus of gpD and the C-terminus of gpShp, was Mg2+-independent and more stable than lambda shp.