Consequences of a biological invasion reveal the importance of mutualism for plant communities.

Seed-dispersal mutualisms have a fundamental role in regenerating natural communities. Interest in the importance of seed dispersal to plant communities has been heightened by worldwide declines in animal dispersers. One view, the 'keystone mutualist hypothesis', predicts that these human-caused losses will trigger a cascade of linked extinctions throughout the community. Implicitly, this view holds that mutualisms, such as seed dispersal, are crucial ecological interactions that maintain the structure and diversity of natural communities. Although many studies suggest the importance of mutualism, empirical evidence for community-level impacts of mutualists has remained anecdotal, and the central role of mutualism, relative to other species interactions, has long been debated in the theoretical literature. Here I report the community-level consequences of a biological invasion that disrupts important seed-dispersal mutualisms. I show that invasion of South African shrublands by the Argentine ant (Linepithema humile) leads to a shift in composition of the plant community, owing to a disproportionate reduction in the densities of large-seeded plants. This study suggests that the preservation of mutualistic interactions may be essential for maintaining natural communities.     

A kinase-regulated PDZ-domain interaction controls endocytic sorting of the beta2-adrenergic receptor.

A fundamental question in cell biology is how membrane proteins are sorted in the endocytic pathway. The sorting of internalized beta2-adrenergic receptors between recycling endosomes and lysosomes is responsible for opposite effects on signal transduction and is regulated by physiological stimuli. Here we describe a mechanism that controls this sorting operation, which is mediated by a family of conserved protein-interaction modules called PDZ domains. The phosphoprotein EBP50 (for ezrinradixin-moesin(ERM)-binding phosphoprotein-50) binds to the cytoplasmic tail of the beta2-adrenergic receptor through a PDZ domain and to the cortical actin cytoskeleton through an ERM-binding domain. Disrupting the interaction of EBP50 with either domain or depolymerization of the actin cytoskeleton itself causes missorting of endocytosed beta2-adrenergic receptors but does not affect the recycling of transferrin receptors. A serine residue at position 411 in the tail of the beta2-adrenergic receptor is a substrate for phosphorylation by GRK-5 (for G-protein-coupled-receptor kinase-5) and is required for interaction with EBP50 and for proper recycling of the receptor. Our results identify a new role for PDZ-domain-mediated protein interactions and for the actin cytoskeleton in endocytic sorting, and suggest a mechanism by which GRK-mediated phosphorylation could regulate membrane trafficking of G-protein-coupled receptors after endocytosis.     

Major Transglycosylation Productsof β-Glucosidase and Their Induction Effects on Cellulase Biosynthesis

以纤维二糖特异诱导培养里氏木霉和黑曲霉菌丝,制备含有细胞外、细胞内和细胞质膜结合态β葡萄糖苷酶的无细胞抽提物,采用色谱持谱联用分析,比较研究他们对纤维二糖的转糖基作用,结果表明两菌株中都只有细胞质膜结合态β葡萄糖苷酶表现出显著的转糖基活性,可检测到的含有β糖苷键葡二糖类的主要转糖基产物是β龙胆二糖而不是槐糖．当在里氏木霉和黑曲霉菌丝的洗脱置换培养体系中加入β龙胆二糖时,可以诱导纤维素酶合成,但它比槐糖（迄今最有潜力的诱导物）的诱导效率低．这些结果支持假说认为是质膜结合态β葡萄糖苷酶从纤维素寡聚物形成转糖基作用产物,作为纤维素酶合成的诱导物