||G protein-coupled receptors (GPCRs) are responsible for the sensations of sight and smell, for regulation of blood pressure and heart rate, and for many other cellular events. Signals impinging upon the exterior of the cell induce a conformational change in these GPCRs that allows them to activate heterotrimeric G proteins within the cell. The activated G proteins then bind to various effectors that initiate downstream cascades, leading to profound physiological change. We study the molecular basis of GPCR-mediated signal transduction, principally via the technique of X-ray crystallography. By determining atomic structures of signaling proteins alone and in complex with their various targets, we can provide important insights into the molecular basis of signal transduction and the disease states that emerge as a result of dysfunctional regulation of these pathways.
G protein-coupled receptor kinases (GRKs)
G protein-coupled receptor kinases (GRKs) are responsible for homologous desensitization of GPCRs, an adaptive process by which activated receptors are rapidly uncoupled from G proteins. The best-characterized member of this family is GRK2, also known as β-adrenergic receptor kinase 1. GRK2 is not only important for myocardiogenesis and regulation of heart contractility but also implicated in the progression of congestive heart failure. In 2003, we reported the atomic structure of GRK2 in a peripheral membrane complex with the heterotrimeric G protein Gβγ. This was the first structure of a GRK and the first of Gβγ in complex with a downstream effector enzyme. Subsequently, we have described the structure of GRK2 alone and the Gαq-GRK2-Gβγ complex (2005). The latter structure was the first of Gαq and a Gαq-effector complex. Gαq is a heterotrimeric G protein involved in smooth muscle function, in regulation of blood pressure and in maladaptive cardiac hypertrophy. The Gαq-GRK2-Gβγ structure also revealed the first glimpse of how activated heterotrimeric G proteins can be arranged at the membrane during active signal transduction and how Gα and Gβγ subunits can convergently interact with a single effector target.
We are also interested in the molecular and biochemical differences between different classes of GRKs. The seven GRKs found in the human genome are classified into three families: GRK2/3, which are ubiquitously expressed; GRK1/7, which play specific roles in phototransduction; and GRK4/5/6, which are all ubiquitously expressed except for GRK4. A distinguishing feature of these families is the structure of their C-terminal domains. We have determined the atomic structure of GRK6 in complex with AMPPNP, a non-hydrolyzable nucleotide analog, as a representative of the GRK4/5/6 family. GRK6 is involved in motor neuron function and thus is a potential drug target for the treatment of Parkinsonís disease. To examine the GRK1/7 family, we have determined the structure of GRK1 in complex with ADP and ATP, and in its apo form. GRK1, also known as rhodopsin kinase, regulates the amplitude of the light response in rod cells. Mutations in GRK1 are responsible for night blindness. One important result from these studies has been to provide what so far has been elusive with GRK2: models of a GRK in different ligand states and the resolution of structural elements believed to be involved in binding GPCRs.
The most well established physiological targets of GRKs are activated GPCRs. GRKs are unique among protein kinases for their ability to recognize only the active form of the receptor. Thus, we believe that GRKs can be used to trap the activated state of a GPCR. Understanding the structure of a GPCR in its activated state is one of the holy grails of modern pharmacology.
Over the course of our studies, we have developed a toolbox of different GRKs that we can produce in abundance and use to probe the molecular determinants of GRK-receptor interaction. Specifically, we are studying how GRK2 interacts with the squid photoreceptor, a Gαq-coupled receptor, and GRK1 with its physiological target rhodopsin. While a crystal structure of these complexes is the most important goal, we are also defining the GPCR binding sites on GRKs with site directed mutagenesis and biochemical assays, cross-linking studies, and co-crystal structures of the intracellular loops of the receptor with GRKs. These studies will further help us define the molecular architecture of signaling complexes that assemble around activated GPCRs.
Heterotrimeric G Protein Regulated Rho Guanine Nucleotide Exchange Factors (RhoGEFs)
GPCRs are also known to be involved in cell transformation, cancer progression and metastasis. One pathway by which this occurs is through the activation of RhoA, a key regulator of cytoskeletal structure and gene transcription. Recently, two families of enzymes responsible for linking these GPCRs to RhoA have been identified. The first family is activated by the G protein Gα12/13 and is critical for platelet activation during wound repair. Of this group, our lab has been studying leukemia-associated RhoGEF (LARG), one of the few RhoGEFs known to be directly responsible for a human cancer. We have determined structures of the catalytic DH/PH domains of LARG alone and in complex with its substrate RhoA, and are in the process of analyzing LARG function using site-directed mutants and either fluorescence-polarization or FRET-based nucleotide exchange assays. We have also determined atomic structures of activated Gα12 and deactivated Gα13 subunits. Future goals are to determine atomic structures of larger fragments of LARG, their complexes with either Gα12 or Gα13, and thereby elucidate the mechanism by which LARG mediates signal transduction from Gα13 to RhoA.
RhoA is also activated by Gαq-coupled receptors via a second family of enzymes represented by p63RhoGEF. We recently published the structure of the Gαq-p63RhoGEF-RhoA complex, capturing a snapshot of three nodes of signal transduction cascade connecting heterotrimeric to small molecular G proteins. Together with the Wieland lab (U. of Heidelberg) and the Miller Lab (Oklahoma Medical Research Foundation), we showed that this pathway is conserved from nematode to humans and that there exists in humans a family of RhoGEFs related to p63RhoGEF that respond to hormones impinging on Gαq-coupled receptors. This family is expected to be at least partly responsible for maladaptive events that occur during heart disease such as cardiac hypertrophy. Current research efforts in the lab are to understand the mechanism by which Gαq activates p63RhoGEF using site-directed mutagenesis and cell-based assays.