Contrary to conventional views, neutral and even acidic amino acids can play crucial roles in NLSs. All regions of the unconventional signal of c-Myc are functionally important. The relative positions of these elements are crucial to the function of these NLSs. The sequence of each added string of amino acids is shown using single-letter amino acid designations. In each case, a string of amino acids was added to pyruvate kinase, a glycolytic enzyme normally found in the cytosol. Nuclear targeting by the single cluster KKKK is dependent on it being preceded by PAA and is stimulated if it is followed by the dipeptide LD. Transcribed image text: Experiments have been performed with the nuclear localization sequence (NLS) of nucleoplasmin. Here, we report that constructs containing an inactive basic cluster downstream of the bipartite signal of nucleoplasmin can be directed to the nucleus by flanking them with specific neutral and acidic residues taken from the signal reported for c-Myc. A different NLS (PAAKRVKLD) has been reported in the oncoprotein c-Myc, but it has received little attention because, unlike other known NLSs, only three of nine residues are basic, and one residue is even acidic. A SwissProt database search shows that more than 50% of nuclear proteins contain a match to this consensus, and many NLSs have since been found to conform to this type of motif in yeast, plants and animals. The nucleoplasmin NLS requires two essential clusters of basic amino acids, separated by a mutation-tolerant spacer (KRPAATKKAGQAKKKK171 ). The NLS of the simian virus 40 large T-antigen (SV40 T-ag) is a single cluster of basic amino acids (PKKKRKV132 single-letter code, the basic amino acids are shown in bold ), whereas the NLS of nucleoplasmin is bipartite. The alpha subunit of importin binds the nuclear localization signal (NLS), and the beta subunit docks at the nuclear pore complex. Nuclear proteins contain information within their primary structures which causes them to accumulate selectively in the nucleus by associating with the cytosolic receptor importin.
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