Danielsen S, Boyd D, Neuhard J
Department of Biological Chemistry, University of Copenhagen, Denmark.
Microbiology (Reading). 1995 Nov;141 ( Pt 11):2905-13. doi: 10.1099/13500872-141-11-2905.
The Escherichia coli codBA operon encodes cytosine permease (CodB) and cytosine deaminase (CodA). CodB mediates uptake of exogenously supplied cytosine, and CodA catalyses the hydrolytic deamination of cytosine to uracil and ammonia. The hydropathic profile of CodB indicates that it is an integral cytoplasmic membrane protein possessing several transmembrane-spanning domains. The membrane topology of CodB was investigated by using gene fusions containing varying lengths of the amino-terminus of CodB fused to either alkaline phosphatase (AP) or beta-galactosidase (BG). The AP activities expressed by the CodB-AP fusions are consistent with a topological model in which the amino- and the carboxy-termini of CodB are located in the cytoplasm, and in which CodB possesses 12 membrane-spanning segments. The enzyme activities of most of the CodB-BG fusions support the model. However, the results obtained with some of the CodB-BG fusions illustrate the limitations of using BG as a reporter protein in studies of membrane protein topology.
大肠杆菌的codBA操纵子编码胞嘧啶通透酶（CodB）和胞嘧啶脱氨酶（CodA）。CodB介导外源提供的胞嘧啶的摄取，而CodA催化胞嘧啶水解脱氨生成尿嘧啶和氨。CodB的亲水性图谱表明它是一种具有多个跨膜结构域的整合细胞质膜蛋白。通过使用含有不同长度的CodB氨基末端与碱性磷酸酶（AP）或β-半乳糖苷酶（BG）融合的基因融合体，研究了CodB的膜拓扑结构。CodB-AP融合体表达的AP活性与一种拓扑模型一致，在该模型中，CodB的氨基末端和羧基末端位于细胞质中，且CodB具有12个跨膜区段。大多数CodB-BG融合体的酶活性支持该模型。然而，一些CodB-BG融合体获得的结果说明了在膜蛋白拓扑结构研究中使用BG作为报告蛋白的局限性。
