分枝杆菌分泌系统

分泌系统对于具有特殊细胞被膜结构的分枝杆菌,尤其是致病性分枝杆菌的存活和毒力非常重要.不少重要的致病因子或存活因子都通过特定的分泌系统进入环境,包括宿主体内.本文从分泌系统的基因、结构组成、分泌底物、转运机制及其与致病菌毒力的关系等几个方面介绍了分枝杆菌(mycobacteria)通用型分泌系统(general secretion pathway,SecA1)、替代型分泌系统(accessory Sec system,SecA2)、双精氨酸分泌系统(twin-arginine translocation,Tat)和Ⅶ型分泌系统(typeⅦsecretion systems,T7S system or ESX)4种分泌系统,并重点分析了Tat分泌系统.这些知识有利于从分泌系统及其底物的角度揭示结核分枝杆菌等胞内致病菌存活和逃避宿主免疫的机理,将为研发新的结核病控制措施提供依据.     

Secretion without Golgi

A growing number of proteins devoid of signal peptides have been demonstrated to be released through the non-classical pathways independent of endoplasmic reticulum and Golgi. Among them are two potent proangiogenic cytokines FGF1 and IL1alpha. Stress-induced transmembrane translocation of these proteins requires the assembly of copper-dependent multiprotein release complexes. It involves the interaction of exported proteins with the acidic phospholipids of the inner leaflet of the cell membrane and membrane destabilization. Not only stress, but also thrombin treatment and inhibition of Notch signaling stimulate the export of FGF1. Non-classical release of FGF1 and IL1alpha presents a promising target for treatment of cardiovascular, oncologic, and inflammatory disorders.     

A Gatekeeper Chaperone Complex Directs Translocator Secretion during Type Three Secretion

Many Gram-negative bacteria use Type Three Secretion Systems (T3SS) to deliver effector proteins into host cells. These protein delivery machines are composed of cytosolic components that recognize substrates and generate the force needed for translocation, the secretion conduit, formed by a needle complex and associated membrane spanning basal body, and translocators that form the pore in the target cell. A defined order of secretion in which needle component proteins are secreted first, followed by translocators, and finally effectors, is necessary for this system to be effective. While the secreted effectors vary significantly between organisms, the ∼20 individual protein components that form the T3SS are conserved in many pathogenic bacteria. One such conserved protein, referred to as either a plug or gatekeeper, is necessary to prevent unregulated effector release and to allow efficient translocator secretion. The mechanism by which translocator secretion is promoted while effector release is inhibited by gatekeepers is unknown. We present the structure of the Chlamydial gatekeeper, CopN, bound to a translocator-specific chaperone. The structure identifies a previously unknown interface between gatekeepers and translocator chaperones and reveals that in the gatekeeper-chaperone complex the canonical translocator-binding groove is free to bind translocators. Structure-based mutagenesis of the homologous complex in Shigella reveals that the gatekeeper-chaperone-translocator complex is essential for translocator secretion and for the ordered secretion of translocators prior to effectors.     

Secretion and membrane assembly

Cytoplasmic proteins undergo rapid and stable folding which buries their apolar segments. In contrast, precursors of secreted and membrane proteins have apolar segments which are recognized by chaperones and membrane receptors to distinguish them from soluble proteins.     

Secretion of bacterial ribonuclease

The investigation of the effect of some components of the medium on the distribution of the secretory guanyl-specific ribonuclease of Bacillus intermedius (EC 3.1.4.23) among various cell fractions and culture liquid showed that the amount of this enzyme in the culture liquid does not depend on the concentration of calcium ions in the medium (within 1-5 mM). The study of the effect of the amino acid substitutions Trp34Asn and Trp70Asn in the ribonuclease molecule showed that the secretion of ribonuclease depends on the formation rate of its secondary structure. The amino acid substitution Trp34Asn completely inhibits ribonuclease secretion.     

Secretion of sulfated thyroglobulin

Thyroid follicle cells from various mammalian species incorporate 35-SO4(2-). Light and electron microscopic autoradiographs show that the Golgi complex is the predominant site of sulfate incorporation and that the secretory product accumulating in the follicle lumen is sulfated. In order to determine which components of the luminal content carry the sulfate residues, inside-out follicles from pig thyroid glands were incubated in the presence of 35-SO4(2-) and the secretory product released into the culture medium was analyzed by polyacrylamide gel electrophoresis. The observations show that the secretory product consists of sulfated thyroglobulin and that approximately 13 sulfate residues are bound covalently to 1 molecule of dimeric thyroglobulin. Digestion of 35-SO4(2-)-thyroglobulin with endoglycosidase H removes 20 to 30% of the radioactivity, indicating that the high mannose carbohydrate side chains carry sulfate residues. The complex carbohydrate side chains are apparently free of sulfate since treatment with endoglycosidase D did not alter the sulfate content. About 2/3 of the sulfate is cleaved by hydrolysis with 1 M HCl (5 min, 95 degrees C) indicating the presence of tyrosine sulfate. Part of the sulfate is exposed and presumably located on the surface of the thyroglobulin molecule as suggested by the direct accessibility of 35-SO4(2-)-thyroglobulin to digestion with sulfatases. The sulfate residues contribute to the anionic state of thyroglobulin. It is postulated that the sulfate residues operate in the regulation of thyroglobulin transport in the cell and in the tight packaging of thyroglobulin in the follicle lumen.