Differential expression and subcellular distribution of the mouse metastasis-associated proteins Mta1 and Mta3

The human metastasis-associated gene (MTA1) is overexpressed in cell lines and tissues representing metastatic tumors. Here we report cloning of the m ouse Mta1 as well as a novel structurally related mouse gene, Mta3. The mou se Mta1 protein shares 94 and 59% homology to the human MTA1 and mouse Mta3 proteins, respectively. Northern blotting analysis using an Mta1 cDNA prob e revealed a prevalent 3 kb hybridization signal in all mouse tissues excep t the skeletal muscle while a smaller similar to 1.0 kb mRNA product was al so detected in the heart. Mta3 transcripts (similar to2 kb) were detected i n most tissues with an additional similar to6.2 kb signal detected in the b rain. In vitro transcription/ translation of the full-length Mta1 and Mta3 cDNAs generated products of the expected molecular masses, i.e. 80 and 60 k Da, respectively. To assess subcellular localization, green fluorescence pr otein (GFP)-tagged expression constructs of Mta1 and Mta3 and various delet ion constructs of GFP-Mta1 were transiently expressed in Balb/MK keratinocy tes. GFP-Mta1 was found exclusively in the nucleus while GFP-Mta3 was prese nt in both the nucleus and cytoplasm. Compared to Mta3, the carboxy termina l end of Mta1 contains an additional nuclear localization signal (NLS) and a proline-rich Src homology 3 (SH3) ligand. The results of transient expres sion experiments of various Mta1 fragments containing these domains in diff erent combinations indicated that nuclear localization of Mta1 depended on the presence of at least one NLS and one SH3 binding site. These SH3 ligand s appeared to be functional as they facilitated interaction with the adapto r protein, Grb2, and the Src-family tyrosine kinase, Fyn. (C) 2001 Elsevier Science B.V. All rights reserved.