Effect of the ionic environment on the molecular structure of bacteriophage SPP1 portal protein

Bacteriophage SPP1 portal protein is a large cyclical homo-oligomer compose d of 13 subunits. The solution structure and assembly behavior of this prot ein with high-point rotational symmetry was characterized. The purified pro tein was present as a monodisperse population of 13-mers, named gp6(H), at univalent salt concentrations in the hundred millimolar range (greater than or equal to 250 mM NaCl) or in the presence of bivalent cations in the mil limolar range (greater than or equal to 5 mM MgCl2). Gp6(H) had a slightly higher sedimentation coefficient, ii smaller shape; dependent frictional ra tio, and a higher rate of intersubunit cross-linking in the presence of mag nesium than in its absence. In the absence of bivalent cations and at univa lent salt concentrations below 250 mM, the 13-mer molecules dissociated par tially into stable monomers,named gp6(L). The monomer had a somewhat differ ent shape from the subunit present in the 13-mer, but maintained a defined tertiary structure. The association-dissociation equilibrium was mainly bet ween the monomer and the 13-mer with a minor population of intermediate oli gomers. Their interconversion was strongly influenced by the ionic environm ent. Under physiological conditions, the concentration of Mg2+ found in the Bacillus subtilis cytoplasm (10-50 mM) probably promotes complete associat ion of gp6 into 13-mer rings with a compact conformation.