The carboxyl- and amino-terminal ends of streptavidin are near the site of
protein-protein contacts in two-dimensional streptavidin crystals. The role
of these C- and N-terminal residues in determining the pH-dependent phase
behavior of crystallization has been investigated with site-directed trunca
tion mutants. Commercial streptavidin (consisting primarily of amino acids
14-136) and two recombinant streptavidin forms, spanning residues 13-136 an
d 13-139, have been crystallized at pH 4-7. The commercial 14-136 protein c
rystallizes in three distinct lattice symmetries, P1, P2, and C-222, respec
tively, depending on pH. The 13-136 mutant also crystallizes in three disti
nct lattices, but with a shifted pH profile that is attributed to the N-ter
minal residue. The presence of amino acids 137-139 inhibits the growth of c
rystals with P1 symmetry at low pH. In addition, we observe a solid-solid p
hase transition in situ from the P2 to the P1 crystal forms for the 13-136
recombinant protein at pH 5.2. We also demonstrate the ability of Brewster
angle microscopy to distinguish between different crystal forms if protein
monolayer densities are sufficiently different.