Tid1 Isoforms Are Mitochondrial DnaJ-like Chaperones with Unique Carboxyl Termini That Determine Cytosolic Fate

Abstract

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 nonmitochondrial 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 nonmitochondrial 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 nonmitochondrial pathways.Tid1 is a human homolog of bacterial DnaJ and the Tid56 tumor suppressor of Drosophila melanogaster. DnaJ-like proteins function as co-chaperones with DnaK-like ATPases to promote the folding, translocation, and/or degradation of polypeptides (1-3). DnaJ-and DnaKlike proteins are highly conserved, and homologs are found in nearly every compartment of the eukaryotic cell and in several tumor viruses (4). In Drosophila, the absence of Tid56 results in abnormal differentiation and morphogenesis, giving rise to Tumorous imaginal discs that lead to lethality during early development (5). The mechanism underlying Tid56 function is unknown.In humans, two alternatively spliced forms of Tid1 are expressed, Tid1-long (Tid1-L) and Tid1-short (Tid1-S), that differ only at their carboxyl-terminal tails (Fig. 1A) (6). Tid1-L contains 33 amino acids unique to its carboxyl terminus, whereas Tid1-S has 6 amino acids (Fig. 1A). Both Tid1-L and -S have a predicted amino-terminal mitochondrial targeting sequence as well as the signature J-domain carrying an HPD motif required for stimulating the ATPase activity of eukaryotic DnaK-like chaperones (1-3). Experiments in mice show that Tid1 is critical for early mammalian development (7). Mice specifically deficient in Tid1 in the heart develop dilated cardiomyopathy, progre...

Reactivation of protein aggregates by mortalin and Tid1—the human mitochondrial Hsp70 chaperone system

“…It was shown that Tid1-L is much more stable in the cytosol than Tid1-S, while both were equally stable inside the mitochondria (Lu et al 2006). Thus, it was very interesting to examine whether Tid1-S and Tid1-L exhibit different reactivation activities.…”

“…4. Since a previous study showed that both Tid1-L and Tid1-S are equally able to complement deletion of yeast Mdj1, we initially focused on studying Tid1-L (Lu et al 2006). …”

“…Moreover, It was demonstrated that expression of the long variant increased apoptosis, while expression of the short variant suppressed apoptosis (Syken et al 1999). Interestingly, Tid1-L was shown to be more stable in the cytosol as compared with Tid1-S, with a longer cytosolic residency time prior to mitochondrial import (Lu et al 2006).…”

“…Several lines of evidence indicate that both Tid1 isoforms are functional homologues of Mdj1: (1) they contain the characteristic Jdomain of the 40-kDa heat shock protein (Hsp40) family; (2) both isoforms were co-immunoprecipitated with mortalin (Syken et al 1999); and (3) Tid1-L and Tid1-S were both able to compensate for an Mdj1 deletion in yeast. This capacity was dependent on the presence of an intact Jdomain and a mitochondrial targeting sequence (Lu et al 2006). Tid1 isoforms were also found to be involved in extra-mitochondrial protein interactions and functions (see below).…”

“…These functions include regulation of cell death, proliferation, and signal transduction (Mitra et al 2009;Syken et al 1999). It was shown that the two isoforms differ in their non-mitochondrial functions due to differences in their abilities to interact with cytosolic and nuclear proteins (Lu et al 2006). Moreover, It was demonstrated that expression of the long variant increased apoptosis, while expression of the short variant suppressed apoptosis (Syken et al 1999).…”