1887

Abstract

The conserved eukaryotic DNA replication protein ORC1 is one of the constituents of pre-replication complexes that assemble at or very near origins prior to replication initiation. ORC1 has been shown to be constitutively nuclear in . This study investigates the sequences involved in nuclear localization of ORC1 in , the causative agent of visceral leishmaniasis. Nuclear localization signals (NLSs) have been reported in only a few proteins. Functional analyses have delineated NLSs to regions of ~60 amino acids in length in the tyrosyl DNA phosphodiesterase I and type II DNA topoisomerase of , and in the kinesin KIN13-1. Using a panel of site-directed mutations we have identified a sequence essential for nuclear import of LdORC1. This sequence at the N terminus of the protein comprises residues 2–5 (KRSR), with K2, R3 and R5 being crucial. Independent mutation of the K2 residue causes exclusion of the protein from the nucleus, while mutating the R5 residue leads to diffusion of the protein throughout the cell. This sequence, however, is insufficient for targeting a heterologous protein (β-galactosidase) to the nucleus. Analysis of additional ORC1 mutations and reporter constructs reveals that while the highly basic tetra-amino acid sequence at the N terminus is essential for nuclear localization, the ORC1 NLS in its entirety is more complex, and of a distributive character. Our results suggest that nuclear localization signalling sequences in nuclear proteins are more complex than what is typically seen in higher eukaryotes.

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2012-07-01
2019-10-18
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