Tid1 isoforms are mitochondrial DnaJ-like chaperones with unique carboxyl termini that determine cytosolic fate

Tid1 is a human homolog of bacterial DnaJ and the Drosophila tumor suppressor Tid56 that has two alternatively spliced isoforms, Tid1-long and -short (Tid1-L and -S), which differ only at their carboxyl termini. Although Tid1 proteins localize overwhelmingly to mitochondria, published data demonstrate principally nonmito-chondrial protein interactions and activities. This study was undertaken to determine whether Tid1 proteins function as mitochondrial DnaJ-like chaperones and to resolve the paradox of how proteins targeted primarily to mitochondria function in nonmito-chondrial pathways. Here we demonstrate that Tid1 isoforms exhibit a conserved mitochondrial DnaJ-like function substituting for the yeast mitochondrial DnaJ-like protein Mdj1p. Like Mdj1p, Tid1 localizes to human mitochondrial nucleoids, which are large protein complexes bound to mitochondrial DNA. Unlike other DnaJs, Tid1-L and -S form heterocomplexes; both unassembled and complexed Tid1 are observed in human cells. Results demonstrate that Tid1-L has a longer residency time in the cytosol prior to mitochondrial import as compared with Tid1-S; Tid1-L is also significantly more stable in the cytosol than Tid1-S, which is rapidly degraded. The longer cytosolic residency time and the half-life of Tid1-L are explained by its interaction with cytosolic Hsc70 and potential protein substrates such as the STAT1 and STAT3 transcription factors. We show that the unique carboxyl terminus of Tid1-L is required for interaction with Hsc70 and STAT1 and -3. We propose that the association of Tid1 with chaperones and/or protein substrates in the cytosol provides a mechanism for the alternate fates and functions of Tid1 in mitochondrial and nonmito-chondrial pathways.