Thornton Jeremy, Blakey Dan, Scanlon Elizabeth, Merrick Mike
Department of Molecular Microbiology, John Innes Centre, Norwich, UK.
FEMS Microbiol Lett. 2006 May;258(1):114-20. doi: 10.1111/j.1574-6968.2006.00202.x.
The Escherichia coli ammonia channel protein, AmtB, is a homotrimeric polytopic inner membrane protein in which each subunit has 11 transmembrane helices. We have shown that the structural gene amtB encodes a preprotein with a signal peptide that is cleaved off to produce a topology with the N-terminus in the periplasm and the C-terminus in the cytoplasm. Deletion of the signal peptide coding region results in significantly lower levels of AmtB accumulation in the membrane but modification of the signal peptidase cleavage site, leading to aberrant cleavage, does not prevent trimer formation and does not inactivate the protein. The presence of a signal peptide is apparently not a conserved feature of all prokaryotic Amt proteins. Comparison of predicted AmtB sequences suggests that while Amt proteins in Gram-negative organisms utilize a signal peptide, the homologous proteins in Gram-positive organisms do not.
大肠杆菌氨通道蛋白AmtB是一种同三聚体多跨膜内膜蛋白，其中每个亚基有11个跨膜螺旋。我们已经表明，结构基因amtB编码一种带有信号肽的前体蛋白，该信号肽被切割掉以产生一种拓扑结构，其N端位于周质，C端位于细胞质。信号肽编码区的缺失导致膜中AmtB积累水平显著降低，但信号肽酶切割位点的修饰导致异常切割，并不妨碍三聚体形成，也不会使该蛋白失活。信号肽的存在显然不是所有原核Amt蛋白的保守特征。预测的AmtB序列比较表明，革兰氏阴性菌中的Amt蛋白利用信号肽，而革兰氏阳性菌中的同源蛋白则不利用。
