Owen P, Meehan M, de Loughry-Doherty H, Henderson I
Department of Microbiology, Moyne Institute of Preventive Medicine, Trinity College Dublin, Ireland.
FEMS Immunol Med Microbiol. 1996 Dec 1;16(2):63-76. doi: 10.1111/j.1574-695X.1996.tb00124.x.
Escherichia coli contains at least two phase-variable proteins in its outer membrane. One, termed antigen 43 (Ag43), is the product of the metastable flu gene located at min 43.6 on the E. coli chromosome and is responsible for colony form variation and for autoaggregation in liquid media. Ag43 is composed of two proteinaceous subunits, alpha 43 and beta 43 in 1:1 stoichiometry. alpha 43 (apparent M(r) 60,000) is surface expressed, extends beyond the O-side chains of smooth lipopolysaccharide and is bound to the cell surface through an interaction with beta 43 (apparent M(r) 53,000), itself an integral, heat-modifiable, outer membrane protein. alpha 43 shows limited N-terminal sequence homology with certain enterobacterial adhesins, and notable sequence homology with AIDA-1, an adhesin of diffuse-adhering E. coli. In addition, alpha 43 contains an RGD motif and a consensus sequence for an (autoproteolytic?) aspartyl protease active site. Expression of Ag43 is subject to reversible phase variation-in liquid minimal medium, the rates of variation from Ag43+ to Ag43- states and from Ag43- to Ag43+ states being approximately 2.2 x 10(-3) and approximately 1 x 10(-3), respectively. Phase switching of Ag43 is regulated by DNA methylation (deoxyadenosine methylase (dam) mutants being 'locked OFF') and by OxyR (oxyR mutants being 'locked ON'). It is proposed that OxyR acts as a repressor of Ag43 transcription by binding to unmethylated GATC sites in the regulatory region of the gene. In some strains, Ag43 may also undergo antigenic variation. A 94 kDa immunocrossreactive outer membrane protein, showing similar rates of phase variation, has additionally been detected for some enteropathogenic and uropathogenic strains of E. coli. This 94 kDa protein can be proteolytically cleaved in situ with trypsin to yield two membrane-bound products with M(r)s and properties similar to those of alpha 43 and beta 43. Results suggest that Ag43 may represent one of a family of antigenically-related high-M(r) adhesins which are synthesized as polyprotein precursors. Some members may be processed and presented on the cell surface as bipartite protein complexes (as Ag43). Others can remain uncleaved.
大肠杆菌外膜中至少含有两种相变蛋白。其中一种称为抗原43（Ag43），是位于大肠杆菌染色体43.6分钟处的亚稳flu基因的产物，负责菌落形态变异以及在液体培养基中的自聚集。Ag43由两个蛋白质亚基组成，α43和β43的化学计量比为1:1。α43（表观分子量60,000）在表面表达，延伸超出光滑脂多糖的O侧链，并通过与β43（表观分子量53,000）相互作用而结合到细胞表面，β43本身是一种整合的、热可修饰的外膜蛋白。α43与某些肠杆菌粘附素的N端序列同源性有限，但与弥漫粘附性大肠杆菌的粘附素AIDA-1有显著的序列同源性。此外，α43含有一个RGD基序和一个（自蛋白水解？）天冬氨酸蛋白酶活性位点的共有序列。Ag43的表达会发生可逆的相变——在液体基本培养基中，从Ag43+状态变为Ag43-状态以及从Ag43-状态变为Ag43+状态的速率分别约为2.2×10-3和约1×10-3。Ag43的相变受DNA甲基化调节（脱氧腺苷甲基化酶（dam）突变体处于“锁定关闭”状态）以及OxyR调节（oxyR突变体处于“锁定开启”状态）。有人提出，OxyR通过结合基因调控区域中未甲基化的GATC位点，作为Ag43转录的阻遏物。在一些菌株中，Ag43也可能发生抗原变异。还在一些致病性大肠杆菌和尿路致病性大肠杆菌菌株中检测到一种94 kDa的免疫交叉反应性外膜蛋白，其相变速率相似。这种94 kDa的蛋白可以用胰蛋白酶原位蛋白水解切割，产生两种膜结合产物，其分子量和性质与α43和β43相似。结果表明，Ag43可能代表一类抗原相关的高分子量粘附素家族中的一员，这些粘附素作为多蛋白前体合成。一些成员可能被加工并以二元蛋白复合物形式（如Ag43）呈现在细胞表面。其他成员可能保持未切割状态。
