Lee Bulla Jr

Professor - Biological Sciences
bulla@utdallas.edu
972-883-4226
FO 3.704B
ORCID
Tags: Microbiology Biology

Professional Preparation

Postdoctoral Research Associate
Northern Regional Research Laboratory, USDA, Peoria, IL - 1969
PhD - Microbiology
Oregon State University - 1968
B.S. - Biology
Midwestern State University - 1965

Research Areas

Research Interests
My research involves structural and functional characterization of insecticidal protein toxins (Cry toxins) produced by the soil bacterium Bacillus thuringiensis(Bt) and their attendant cadherin G protein-coupled receptors (GPCRs).  Thesecadherin GPCRsare located in the midgut epithelium of susceptible insects. Several GPCRs, categorized by the presence of seven membrane-spanning α-helical segments separated by alternating intracellular and extracellular loop regions, have been investigated in mammalian systems.  However, little is known about insect cadherin GPCRs that bind Cry toxins.  Particularly, the structural basis of their function and participation in GPCR signaling is not well understood.

The overall objective of my research is to determine the structural basis of insect cadherin GPCR function and how interaction of Cry toxins with cadherin receptors dictates the unique functional properties of these single-pass signaling molecules. My laboratory has demonstrated that cadherin GPCR signaling is ligand (Cry toxin)-regulated in the tobacco hornworm (Manduca sexta) and, most likely, is a general phenomenon in other insect systems including beetles and mosquitoes.  The primary focus now is onthe Cry1Ab toxin (kDa = 65.1) produced by Bt, which binds to a conserved structural motif in the 12thectodomain (EC12; kDa = 13.4) of the cadherin receptor BT-R1(kDa~210) present in midgut epithelial cells of tobacco hornworm larvae.  This incident is essential for toxicity of Cry1Ab to the insect.  Binding of toxin to BT-R1triggers a signaling event, which stimulates its coupled heterotrimeric G protein.  The G protein, in turn, activates an effector enzyme, adenylyl cyclase,that catalyzes a dramatic increase in theintracellular second messengercAMP.  cAMP activates protein kinase A, bringing about cytological rearrangement and ion fluxing as well as other significant changes such as enhanced exocytosis of BT-R1from intracellular membrane vesicles to the cell surface.  The consequent enrichment of BT-R1on the cell surface is mediated by Cry1Ab-induced signal transduction, and amplification of the signaling sets the stage for additional toxin binding and increased killing.  Cry1Ab toxin binding to BT-R1in cultured insect cells is specific and selective, and toxin action is inhibited by blocking the binding of Cry1Ab to BT-R1in either cultured cells or whole insects. Indeed, the toxin-binding region EC12, the proximal cadherin repeat of the BT-R1ectodomain, completely inhibits Cry1Ab toxin action in insects when it is pre-incubated with the toxin at a 1:1 molar ratio.  Moreover, BT-R1toxicity of High Five cells transfected with BT-R1cDNA is inhibited maximally when soluble EC12 is present at a 1:1 ratio with Cry1Ab.

Based on our cytological and biochemical investigations, we believe that univalent binding of the Cry1Ab toxin affects the structural and functional properties of BT-R1to bring about cellular damage and total loss of function in midgut epithelial cells of tobacco hornworm larvae exposed to the toxin.  Structural studies currently underway should provide further insight into the molecular mechanisms of cadherin GPCRactivation by Cry toxins and constitutive activity.

Publications

Expression in Spodoptera frugiperda (Sf21) insect cells of BT-R(1), a cadherin-related receptor from Manduca sexta for Bacillus thuringiensis Cry1Ab toxin. 2001 - Journal Article
Effects of midgut-protein-preparative and ligand binding procedures on the toxin binding characteristics of BT-R1, a common high-affinity receptor in Manduca sexta for Cry1A Bacillus thuringiensis toxins. 1998 - Journal Article
Southern analysis of BT-R1, the Manduca sexta gene encoding the receptor for the Cry1Ab toxin of Bacillus thuringiensis. 1997 - Journal Article
Ligand specificity and affinity of BT-R1, the Bacillus thuringiensis Cry1A toxin receptor from Manduca sexta, expressed in mammalian and insect cell cultures. 1997 - Journal Article
Further characterization of BT-R1, the cadherin-like receptor for Cry1Ab toxin in tobacco hornworm (Manduca sexta) midguts. 1997 - Journal Article
Cloning and expression of a receptor for an insecticidal toxin of Bacillus thuringiensis. 1995 - Journal Article
A specific binding protein from Tenebrio molitor for the insecticidal toxin of Bacillus thuringiensis subsp. tenebrionis. 1994 - Journal Article
Insertion sequence elements in Bacillus thuringiensis subsp. darmstadiensis. 1993 - Journal Article
A specific binding protein from Manduca sexta for the insecticidal toxin of Bacillus thuringiensis subsp. berliner. 1993 - Journal Article
Larvicidal activity of chimeric Bacillus thuringiensis protoxins. 1990 - Journal Article

Appointments

Assistant Professor (Adjunct), Department of Biology
Bradley University [1969–1973]
Project Leader, Section of Microbiology
USDA [1968–1973]
Agriculture Research Service, USDA