Tolerico L H, Benko A L, Aris J P, Stanford D R, Martin N C, Hopper A K
Program in Cell and Molecular Biology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
Genetics. 1999 Jan;151(1):57-75. doi: 10.1093/genetics/151.1.57.
MOD5 encodes a tRNA modification activity located in three subcellular compartments. Alternative translation initiation generates Mod5p-I, located in the mitochondria and the cytosol, and Mod5p-II, located in the cytosol and nucleus. Here we study the nucleus/cytosol distribution of overexpressed Mod5p-II. Nuclear Mod5p-II appears concentrated in the nucleolus, perhaps indicating that the nuclear pool may have a different biological role than the cytoplasmic and mitochondrial pools. Mod5p contains three motifs resembling bipartite-like nuclear localization sequences (NLSs), but only one is sufficient to locate a passenger protein to the nucleus. Mutations of basic residues of this motif cumulatively contribute to a cytosolic location for the fusion proteins. These alterations also cause decreased nuclear pools of endogenous Mod5p-II. Depletion of nuclear Mod5p-II does not affect tRNATyr function. Despite the NLS, most Mod5p is cytosolic. We assessed whether Mod5p sequences cause a karyophilic reporter to be located in the cytosol. By this assay, Mod5p may contain more than one region that functions as cytoplasmic retention and/or nuclear export sequences. Thus, distribution of Mod5p results from the presence/absence of mitochondrial targeting information and sequences antagonistic for nuclear and cytosolic locations. Mod5p is highly conserved; sequences responsible for subcellular distribution appear to reside in "accessory" motifs missing from prokaryotic counterparts.
MOD5编码一种位于三个亚细胞区室的tRNA修饰活性。选择性翻译起始产生位于线粒体和细胞质中的Mod5p-I，以及位于细胞质和细胞核中的Mod5p-II。在此，我们研究过表达的Mod5p-II在细胞核/细胞质中的分布。核内的Mod5p-II似乎集中在核仁中，这可能表明核内池与细胞质和线粒体池可能具有不同的生物学作用。Mod5p包含三个类似于二分体样核定位序列（NLS）的基序，但只有一个足以将一个乘客蛋白定位到细胞核。该基序中碱性残基的突变累积导致融合蛋白定位于细胞质中。这些改变还导致内源性Mod5p-II的核内池减少。核内Mod5p-II的缺失不影响tRNATyr的功能。尽管存在NLS，但大多数Mod5p位于细胞质中。我们评估了Mod5p序列是否会导致亲核报告蛋白定位于细胞质中。通过该检测，Mod5p可能包含不止一个起细胞质保留和/或核输出序列作用的区域。因此，Mod5p的分布是由线粒体靶向信息的有无以及对细胞核和细胞质定位起拮抗作用的序列所导致的。Mod5p高度保守；负责亚细胞分布的序列似乎存在于原核生物对应物中缺失的“辅助”基序中。
