A crystallizable form of the Streptococcus gordonii surface antigen SspB C-domain obtained by limited proteolysis

In Situ Proteolysis for Crystallization of Membrane Bound Cytochrome P450 17A1 and 17A2 Proteins from Zebrafish

Fish and human cytochrome P450 (P450) 17A1 catalyze both steroid 17α-hydroxylation and 17α,20-lyase reactions. Fish P450 17A2 catalyzes only 17α-hydroxylation. Both enzymes are microsomal-type P450s, integral membrane proteins that bind to the membrane through their N-terminal hydrophobic segment, the signal anchor sequence. The presence of this N-terminal region renders expression of full-length proteins challenging or impossible. For some proteins, variable truncation of the signal anchor sequence precludes expression or results in poor expression levels. To crystallize P450 17A1 and 17A2 in order to gain insight into their different activities, we used an alternative N-terminal sequence to boost expression together with in situ proteolysis. Key features of our approach to identify crystallizable P450 fragments were the use of an N-terminal leader sequence, a screen composed of 12 proteases to establish optimal cleavage, variations of protease concentration in combination with an SDS-PAGE assay, and analysis of the resulting fragments using Edman sequencing. Described in this unit are protocols for vector preparation, expression, purification, and in situ proteolytic crystallization of two membrane-bound P450 proteins.

Two intramolecular isopeptide bonds are identified in the crystal structure of the Streptococcus gordonii SspB C-terminal domain

Streptococcus gordonii is a primary colonizer and is involved in the formation of dental plaque. This bacterium expresses several surface proteins. One of them is the adhesin SspB, which is a member of the Antigen I/II family of proteins. SspB is a large multi-domain protein that has interactions with surface molecules on other bacteria and on host cells, and is thus a key factor in the formation of biofilms. Here, we report the crystal structure of a truncated form of the SspB C-terminal domain, solved by single-wavelength anomalous dispersion to 1.5 A resolution. The structure represents the first of a C-terminal domain from a streptococcal Antigen I/II protein and is comprised of two structurally related beta-sandwich domains, C2 and C3, both with a Ca(2+) bound in equivalent positions. In each of the domains, a covalent isopeptide bond is observed between a lysine and an asparagine, a feature that is believed to be a common stabilization mechanism in Gram-positive surface proteins. S. gordonii biofilms contain attachment sites for the periodontal pathogen Porphyromonas gingivalis and the SspB C-terminal domain has been shown to have one such recognition motif, the SspB adherence region. The motif protrudes from the protein, and serves as a handle for attachment. The structure suggests several additional putative binding surfaces, and other binding clefts may be created when the full-length protein is folded.