Nuclear Magnetic Resonance Characterization of the Type III Secretion System Tip Chaperone Protein PcrG of Pseudomonas aeruginosa

The type III secretion system needle, tip, and translocon

Many Gram-negative bacteria pathogenic to plants and animals deploy the type III secretion system (T3SS) to inject virulence factors into their hosts. All bacteria that rely on the T3SS to cause infectious diseases in humans have developed antibiotic resistance. The T3SS is an attractive target for developing new antibiotics because it is essential in virulence, and part of its structural component is exposed on the bacterial surface. The structural component of the T3SS is the needle apparatus, which is assembled from over 20 different proteins and consists of a base, an extracellular needle, a tip, and a translocon. This review summarizes the current knowledge on the structure and assembly of the needle, tip, and translocon.

Concomitant disorder and high-affinity zinc binding in the human zinc- and iron-regulated transport protein 4 intracellular loop

The human zinc- and iron-regulated transport protein 4 (hZIP4) protein is the major plasma membrane protein responsible for the uptake of zinc in the body, and as such it plays a key role in cellular zinc homeostasis. hZIP4 plasma membrane levels are regulated through post-translational modification of its large, disordered, histidine-rich cytosolic loop (ICL2) in response to intracellular zinc concentrations. Here, structural characteristics of the isolated disordered loop region, both in the absence and presence of zinc, were investigated using nuclear magnetic resonance (NMR) spectroscopy. NMR chemical shifts, coupling constants and temperature coefficients of the apoprotein, are consistent with a random coil with minor propensities for transient polyproline Type II helices and β-strand in regions implicated in post-translational modifications. The ICL2 protein remains disordered upon zinc binding, which induces exchange broadening. Paramagnetic relaxation enhancement experiments reveal that the histidine-rich region in the apoprotein makes transient tertiary contacts with predicted post-translational modification sites. The residue-specific data presented here strengthen the relationship between hZIP4 post-translational modifications, which impact its role in cellular zinc homeostasis, and zinc sensing by the intracellular loop. Furthermore, the zinc sensing mechanism employed by the ICL2 protein demonstrates that high-affinity interactions can occur in the presence of conformational disorder.

Isotope-labeling strategies for solution NMR studies of macromolecular assemblies

Proteins come together in macromolecular assemblies, recognizing and binding to each other through their structures, and operating on their substrates through their motions. Detailed characterization of these processes is particularly suited to NMR, a high-resolution technique sensitive to structure, dynamics, and interactions. Advances in isotope-labeling have enabled such studies to an ever-increasing range of systems. Here we highlight recent applications and bring to the fore the range of options to produce labeled proteins and to control the specific placement of isotopes. The increased labeling control and affordability, together with the possibility to combine strategies will further deepen and extend the range of protein assembly investigations.