Subcellular localization of ribosomal P0-like protein MRT4 is determined by its N-terminal domain

The Mrt4 protein, showing extensive sequence similarity to the ribosomal PO protein, is classified as a ribosomal PO-like protein and acts as a trans-acting factor which modulates the assembly of the pre-60S particle. In this report we investigated the biological nature of the human Mrt4 protein. First, we constructed a series of hybrid hMrt:4-P0 proteins by replacing various domains of the PO protein with corresponding protein fragments from hMrt4. We found that hMrt4 binds to the same site on the large ribosomal subunit as does PO, but despite the sequence homology it is not able to functionally complement the lack of PO. Using fluorescence microscopy and biochemical approaches we also show that hMrt4 occupies predominantly the nucleolar compartment, in contrast to PO and P1/P2, which are located in the cytoplasm. The nucleolar accumulation of hMrt4 does not depend on a specific nucleolus localization signal, but rather occurs via interaction with established nucleolar components such as rRNA; however, nuclear import of hMrt4 is dependent on a short sequence in the N-terminal part of the protein. Functional analysis with specific inhibitors, actinomycin D and leptomycin B, showed that hMrt4 is a trans-acting factor involved in ribosome maturation, with nucleus-cytoplasm shuttling capacity. (C) 2010 Elsevier Ltd. All rights reserved.