Prinz W A, Spiess C, Ehrmann M, Schierle C, Beckwith J
Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115, USA.
EMBO J. 1996 Oct 1;15(19):5209-17.
Most extracytoplasmic proteins are synthesized with an N-terminal signal sequence that targets them to the export apparatus. Escherichia coli prlA mutants (altered in the secY gene) are able to export cell envelope proteins lacking any signal sequence. In order to understand how such proteins are targeted for export, we isolated mutations in a signal sequenceless version of alkaline phosphatase that block its export in a prlA mutant. The mutations introduce basic amino acyl residues near the N-terminus of alkaline phosphatase. These changes do not disrupt an N-terminal export signal in this protein since the first 25 amino acids can be removed without affecting its export competence. These findings suggest that signal sequenceless alkaline phosphatase does not contain a discrete domain that targets it for export and may be targeted simply because it remains unfolded in the cytoplasm. We propose that basic amino acids near the N-terminus of a signal sequenceless protein affect its insertion into the translocation apparatus after it has been targeted for export. These findings allow the formulation of a model for the entry of proteins into the membrane-embedded export machinery.
大多数胞外蛋白在合成时带有一个N端信号序列，该序列将它们导向输出装置。大肠杆菌prlA突变体（secY基因发生改变）能够输出缺乏任何信号序列的细胞膜蛋白。为了了解此类蛋白是如何被导向输出的，我们在无信号序列的碱性磷酸酶版本中分离出了一些突变，这些突变会在prlA突变体中阻断其输出。这些突变在碱性磷酸酶的N端附近引入了碱性氨酰基残基。这些变化不会破坏该蛋白中的N端输出信号，因为前25个氨基酸可以被去除而不影响其输出能力。这些发现表明，无信号序列的碱性磷酸酶不包含一个将其导向输出的离散结构域，可能仅仅因为它在细胞质中保持未折叠状态而被导向输出。我们提出，无信号序列蛋白N端附近的碱性氨基酸在其被导向输出后会影响其插入转运装置。这些发现使得能够构建一个蛋白质进入膜嵌入输出机制的模型。
