Role of N- and C-Terminal Amino Acids in Two-Dimensional Streptavidin Crystal Formation

Abstract

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 truncation mutants. Commercial streptavidin (consisting primarily of amino acids 14−136) and two recombinant streptavidin forms, spanning residues 13−136 and 13−139, have been crystallized at pH 4−7. The commercial 14−136 protein crystallizes in three distinct lattice symmetries, P1, P2, and C222, respectively, depending on pH. The 13−136 mutant also crystallizes in three distinct lattices, but with a shifted pH profile that is attributed to the N-terminal residue. The presence of amino acids 137−139 inhibits the growth of crystals with P1 symmetry at low pH. In addition, we observe a solid−solid phase transition in situ from the P2 to the P1 crystal forms for the 13−136 recombinant protein at pH 5.2. We also demonst...