Chamankhah M, Fontanie T, Xiao W
Department of Microbiology and Immunology, University of Saskatchewan, Saskatoon, Canada.
Genetics. 2000 Jun;155(2):569-76. doi: 10.1093/genetics/155.2.569.
The yeast Mre11 protein participates in important cellular functions such as DNA repair and telomere maintenance. Analysis of structure-function relationships of Mre11 has led to identification of several separation-of-function mutations as well as N- and C-terminal domains essential for Mre11 meiotic and mitotic activities. Previous studies have established that there is a strong correlation between Mre11 DNA repair and telomere maintenance functions and that Mre11-Rad50-Xrs2 complex formation appears to be essential for both of these activities. Here we report that the mre11(ts) allele, previously shown to cause temperature-dependent defects in DNA repair and meiosis, confers a temperature-independent telomere shortening, indicating that mre11(ts) is a separation-of-function mutation with respect to DNA repair and telomere maintenance. In a yeast two-hybrid system, Mre11(ts) fails to form a homodimer or interact with Rad50 and Xrs2 irrespective of experimental temperatures. These observations collectively suggest that the Pro(162)Ser substitution in Mre11(ts) confers a novel separation of Mre11 mitotic functions. Moreover, we observed that while overexpression of the 5'-3' exonuclease gene EXO1 partially complements the MMS sensitivity of mre11, rad50, and xrs2 null mutants, it has no effect on telomere shortening in these strains. This result provides additional evidence on possible involvement of distinctive mechanisms in DNA repair and telomere maintenance by the Mre11-Rad50-Xrs2 complex.
酵母Mre11蛋白参与DNA修复和端粒维持等重要细胞功能。对Mre11结构-功能关系的分析已鉴定出多个功能分离突变以及Mre11减数分裂和有丝分裂活性所必需的N端和C端结构域。先前的研究已证实，Mre11的DNA修复和端粒维持功能之间存在很强的相关性，并且Mre11-Rad50-Xrs2复合物的形成似乎对这两种活性都至关重要。在此我们报告，先前显示会导致DNA修复和减数分裂出现温度依赖性缺陷的mre11(ts)等位基因会导致与温度无关的端粒缩短，这表明mre11(ts)是一种在DNA修复和端粒维持方面具有功能分离的突变。在酵母双杂交系统中，无论实验温度如何，Mre11(ts)都无法形成同二聚体或与Rad50和Xrs2相互作用。这些观察结果共同表明，Mre11(ts)中的Pro(162)Ser取代赋予了Mre11有丝分裂功能的一种新的功能分离。此外，我们观察到，虽然5'-3'核酸外切酶基因EXO1的过表达部分弥补了mre11、rad50和xrs2缺失突变体对甲磺酸甲酯（MMS）的敏感性，但对这些菌株中的端粒缩短没有影响。这一结果为Mre11-Rad50-Xrs2复合物在DNA修复和端粒维持中可能涉及不同机制提供了额外证据。
