The structural basis for molecular recognition
Transmembrane signaling events requires the formation of transient complexes across the membrane. While challenging, the determination of the architecture of these complexes is required for understanding how biological signals are transmitted. The research in the Iverson laboratory uses several model systems to investigate these transmembrane signaling processes, and is concurrently working to develop methods for membrane protein crystallography.
Mechanisms of G protein signaling
GPCRs bind a diverse set of ligands to elicit cellular responses and represent >50% of pharmacological therapeutic targets. A major challenge in the determination of GPCR structures is that these receptors sample the activated and inactivated states in the absence of cognate ligand. This conformational heterogeneity results in basal activity observed in GPCRs. We are developing methods for the expression and stabilization of GPCRs for structure determination.
The rate-determining step of GPCR signaling is nucleotide exchange of GDP for GTP in the Gα subunit of the G protein. Physiologically, a complex between GDP-bound Gαβγ heterotrimer and activated GPCR catalyzes this exchange. In collaboration with Prof. Heidi Hamm (Vanderbilt) we have developed methods for the stabilization of G proteins that represent this activated state as well as stabilization of the GPCR-Gαβγ complex itself.
The Gαβγ heterotrimer shown in a view highlighting the receptor binding site. The α5 helix of the Gα subunit may contribute to receptor catalyzed nucleotide exchange.
The termination of GPCR signaling requires interactions between recpetor and arrestin. In collboration with Vsevold Gurevich (Vanderbilt), we are investigating the structural basis for the receptor-arrestin interaction and the structural basis for arrestin signaling.
The quinol:fumarate reductase respiratory complex – a bioenergetic enzyme and a signaling scaffold
The structure of the E. coli complex II homolog quinol:fumarate reductase (QFR) was the 11th published membrane protein structure. With our long-term collaborator Prof. Gary Cecchini (UCSF VA Medical Center), this research continues to investigate the structure-activity relationships in the QFR enzyme and how this bioenergetic protein works as a signaling scaffold to affect flagellar switching.
Structure of the E. coli QFR. Each of the 4 subunits is highlighted with a different color. We are investigating the importance of movement of the capping domain (circled), and three catalytically-important residues (magenta). Electron transfer cofactors and intercofactor distances are labeled to the left.
Host-pathogen recognition... and vice versa
The interplay between host and pathogen involves reciprocal recognition events. Recent studies have focused on the basis for pathogen binding to platelets during infective endocarditis. In collaboration with the Sullam laboratory at UCSF and the Thomas laboratory at University of Washington, we are investigating the molecular basis for how Serine Rich Repeat adhesins of pathogenic bacteria.
Structure of the Serine Rich Repeat Adhesin GspB from Streptococcus gordonii. GspB forms three domains (CnaA, red, Siglec, yellow, and Unique, cyan). Host carbohydrate binds to the Siglec domain.
Substrate recognition and engineering in enzymes
Successful engineers frequently adapt ideas from nature. For example, Swiss engineer George de Mestral developed Velcro after being inspired by burdock burs sticking to clothes and hair. Like classical engineers, bioengineers adapt ideas from nature, but use biomolecules as building blocks. We are investigating the fundamental mechanisms of substrate recognition in enzymes. In collaboration with the laboratory of Brian Bachmann (Vanderbilt University) we are applying these basic findings on substrate fidelity with evolutionary principles to identify whether there are improved methods for bioengineering enzymes with novel catalytic properties.
The structures of EvdO1 and EvdO2, two putative orthoester synthases. These related enzymes catalyze a 2-electron ring closure on related substrates.
Structure determination of membrane proteins by any method is still challenging. We have worked to improve the methods for stabilization and crystallization of these proteins. Our efforts have resulted in the development of a screen for b-barrel membrane proteins that is currently marketed by Emerald Biosystems. We are further working to identify alternative media for the solubilization of membrane proteins.