Insect Midgut and Insecticidal Proteins

1. Auflage 2014
ISBN: 978-0-12-800330-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band Volume 47, 436 Seiten

Reihe: Advances in Insect Physiology

This volume of Advances in Insect Physiology contains comprehensive interdisciplinary reviews on basic and practical aspects relevant to Insect Midgut and Insecticidal Proteins. - Contains important, comprehensive and in-depth reviews - An essential reference source for invertebrate physiologists and neurobiologists, entomologists, zoologists, and insect biochemists - First published in 1963, this serial is ranked second in the highly competitive ISI category of Entomology

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Chapter Two Diversity of Bacillus thuringiensis Crystal Toxins and Mechanism of Action
Michael J. Adang*,†; Neil Crickmore‡; Juan Luis Jurat-Fuentes§    * Department of Entomology, University of Georgia, Athens, Georgia, USA
† Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia, USA
‡ School of Life Sciences, University of Sussex, Falmer, Brighton, United Kingdom
§ Department of Entomology and Plant Pathology, University of Tennessee, Knoxville, Tennessee, USA Abstract
Parasporal crystals produced by Bacillus thuringiensis (Bt) bacteria are the main virulence factors underlying Bt toxicity to insects. Parasporal crystals are composed primarily of Cry and Cyt proteins that act on the midgut of susceptible insects. Cry proteins are an important component of Bt biopesticides and are vital tools for insect control via expression in transgenic crop plants. Some members of the Cry group are more distantly related including ETX/MTX and binary type toxins. Cry toxin structure and action involves critical steps in toxin activation, binding to receptors such as cadherin and then aminopeptidase or alkaline phosphatase probably in a ‘sequential binding’ manner. Specific Cry toxin–receptor interactions are a focus of this review. Recently, the importance of midgut ATP-binding cassette proteins to Cry intoxication of insects has been demonstrated. Mechanistic details involved in ‘sequential binding’ and ‘pore formation’ models are examined. The Cyt toxin of Bt subspecies israelensis is an important and interesting component in Cry–midgut interactions in mosquitoes. For some Cry toxins, Cyt serves as a receptor for docking to midgut membrane. Recent engineering work has demonstrated that Cyt can be re-targeted generating novel toxins for insect control. Overall, we review the remarkable progress made in the past 20 years in discovering novel Cry toxins and in elucidating complex mechanisms of Cry and Cyt toxin action; subjects relevant to the long-term control of insects that damage crops and vector human disease. Keywords Bacillus thuringiensis Cry toxin Classification Mode of action Cyt toxin Parasporin Midgut receptor Cadherin Aminopeptidase Alkaline phosphatase 1 Introduction
The Gram-positive bacterium Bacillus thuringiensis (Bt) is characterized by the proteinaceous crystals that it synthesizes in the mother cell during sporulation (Aronson et al., 1986; Bulla et al., 1980). The history of the discovery and development of Bt has been extensively reviewed (Beegle and Yamamoto, 1992; Burges, 2001; Jurat-Fuentes and Jackson, 2012). Tens of thousands of Bt strains have been isolated and most Bt strains are active against larval stages of insects. Products based on Bt have been registered as pesticides in the United States since 1961. As with other biological pesticides, Bt offers a number of advantages over synthetic pesticides, including lack of polluting residues, high specificity to target insects and safety to nontarget organisms. Consequently, the broadest uses of Bt are on food crops and in forestry where safety and specific action are desirable. Disadvantages of Bt are its high specificity and low persistence. While Bt is the most successful biopesticide for insect control it remains a small part, about 2%, of the total insecticide market. The widest usage of Bt for insect control is through transgenic plants producing Bt insecticidal proteins, particularly corn and cotton (James, 2009). The presence of a parasporal crystal is the phenotypic trait of Bt and is used to separate this bacterium from other Bacillus species (Vilas-Bôas et al., 2007). The main components of the crystal are the delta-endotoxins, that act as the primary virulence factor for this pathogen (Raymond et al., 2010), although other proteins are also present in the crystal and have a role in toxin and/or crystal structure (Diaz-Mendoza et al., 2012; Staples et al., 2001). Toxins found in the crystal are classified into two families known as Cry and Cyt (Höfte and Whiteley, 1989). The Cry (from crystal) toxins represent a large family currently consisting of around 300 different members (Crickmore et al., 2014). The Cyt toxins are characterized by possessing a general cytolytic activity in vitro, although they show a primarily dipteran-specific activity in vivo (Soberon et al., 2013a). A third family of protein toxins—the Vegetative Insecticidal Proteins (Vips)—are not classified as crystal toxins since they are secreted from vegetatively growing cells rather than included in the crystal during sporulation (de Maagd et al., 2003). The characteristics of Bt crystal toxins will be discussed in Section 2 beginning with how Cry toxins are defined, named and classified into groups. We discuss conserved features of three-domain Cry toxins and the structurally distinct ETX/MTX and binary-like toxins that have Cry designations. We briefly describe Vip toxins with respect to their relationship to Cry toxins. Also in Section 2, we introduce parasporins, which typically have no known insect target, but have toxicity to specific human cell lines (Mizuki et al., 1999). Recent advances and innovations in toxin discovery are presented in Section 2. Recent reviews of mechanisms of three-domain Cry toxin action (Pardo-López et al., 2012; Vachon et al., 2012) have integrated complex events which occur in insect midgut after Cry protoxin processing and upon contact with the target midgut membrane. In Section 3, we discuss Cry toxin structure in the context of the respective role of each structural domain in Cry toxin action. We also discuss the importance of Cry solubilization from crystals and proteolytic processing to host specificity. Receptor molecules on insect midgut such as cadherin, aminopeptidase and alkaline phosphatase (ALP) have been extensively reviewed (Bravo et al., 2011; Pigott and Ellar, 2007), particularly in the context of models of Cry toxin action. In Section 4, we discuss those receptor molecules with an emphasis on combinations of Cry toxins and insect systems where receptor function has been established. We also include in the discussion evidence demonstrating a critical role in toxin action for ATP-binding cassette (ABC) proteins (Heckel, 2012), and the relevance of glycolipids and other midgut molecules to toxin action. Models of Cry toxin action describing events occurring between Cry-binding midgut and pore formation are extant in recent literature (Bravo et al., 2004, 2011). In Section 5, we examine some of the mechanistic details involved in the ‘sequential binding’ and ‘pore formation’ models of toxin action. The Cyt toxin of Bt subsp. israelensis is a major crystal component and is critical to mosquitocidal toxicity (Ben-Dov, 2014). Since an excellent recent review summarizes the mechanism of Cyt toxin action (Soberon et al., 2013b), we will briefly describe Cyt pore formation, its role as a Cry receptor and recent engineering work that re-targets Cyt toxin resulting in a novel toxin for pest control (Chougule et al., 2013). An excellent review on mechanism of Bt Cry toxin action was previously published in Advances in Insect Physiology (Knowles, 1994) and we focus in this review to provide an update on the remarkable progress made in the past 20 years on the subject. 2 General Characteristics of B. thuringiensis Crystal Toxins
2.1 Definition and classification of crystal toxins
Cry toxins are officially defined as proteins that have significant sequence similarity to existing toxins within the nomenclature or be a B. thuringiensis parasporal inclusion protein that exhibits pesticide activity, or some experimentally verifiable toxic effect, to a target organism (Crickmore et al., 1998). Naming of toxins is based solely on amino acid sequence identity and does not take into account their toxicity; thus, toxins that are active against the same order of insect will not necessarily share any similarity in their names. A toxin's name consists of four levels, e.g., Cry41Ab1, the first number is the primary level and all toxins sharing this first number (41 in the example above) will share significant sequence identity—at least 45%. Toxins sharing primary, secondary and tertiary level descriptors will have increasing sequence identity. Toxins that differ only in the quaternary level descriptor (e.g. Cry41Ab1 and Cry41Ab2) will have at least 95% sequence identity. A different quaternary level descriptor is given to all newly characterized toxins, and as a result some toxins are actually identical to others in the nomenclature, but have been assigned different names. The definition of a Cry toxin as...