Change in the solubility of crystalline Fraction I proteins correlated with change in the composition of the small subunit

Abstract

Fraction I protein crystals were obtained by a simple method from four additional species in addition to seven species of Nicotiana previously reported and from Solanum melongena. Crystals were obtained neither from several other genera of the Solanaceal nor from N. debneyi, but ¹⁴C protein from the latter co-crystallized with N. tabacum Fraction I protein. Co-crystallization did not occur with ¹⁴C proteins from species of Tagetes, Allium, Beta, Brassica and Hyocyamus whose Fraction I proteins were evidently too different in their quaternary structures to occupy the same crystal lattice with N. tabacum protein. Fraction I proteins from N. gossei and N. excelsior differed in solubility as a function of the NaCl concentration. The two proteins were alike in the isoelectric point of the three primary peptides composing the large subunit, but differed in the isoelectric point of one out of four primary peptides of the small subunit; this difference was also consistent with a difference in tryptic peptide fingerprints. Proteins from N. tabacum and N. glauca differed both in the composition of their large and small subunits but did not differ in solubility. However, by changing the composition of the small subunit without changing the large subunit, the solubility of each protein was changed. The change in small subunit composition could be achieved by isolating proteins from the reciprocal F₁ hybrids of N. tabacum × N. glauca where the maternal inheritance regulates the composition of the large subunit, whereas both maternal and paternal genes regulate the composition of the small subunit.