Unraveling the Intricate Organization of Mammalian Mitochondrial Presequence Translocases: Existence of Multiple Translocases for Maintenance of Mitochondrial Function

Abstract

Mitochondria are indispensable organelles implicated in multiple aspects of cellular processes, including tumorigenesis. Heat shock proteins play a critical regulatory role in accurately delivering the nucleus-encoded proteins through membrane-bound presequence translocase (Tim23 complex) machinery. Although altered expression of mammalian presequence translocase components had been previously associated with malignant phenotypes, the overall organization of Tim23 complexes is still unsolved. In this report, we show the existence of three distinct Tim23 complexes, namely, B1, B2, and A, involved in the maintenance of normal mitochondrial function. Our data highlight the importance of Magmas as a regulator of translocase function and in dynamically recruiting the J-proteins DnaJC19 and DnaJC15 to individual translocases. The basic housekeeping function involves translocases B1 and B2 composed of Tim17b isoforms along with DnaJC19, whereas translocase A is nonessential and has a central role in oncogenesis. Translocase B, having a normal import rate, is essential for constitutive mitochondrial functions such as maintenance of electron transport chain complex activity, organellar morphology, iron-sulfur cluster protein biogenesis, and mitochondrial DNA. In contrast, translocase A, though dispensable for housekeeping functions with a comparatively lower import rate, plays a specific role in translocating oncoproteins lacking presequence, leading to reprogrammed mitochondrial functions and hence establishing a possible link between the TIM23 complex and tumorigenicity. N ormal cellular function requires homeostatic counterbalance of various metabolic pathways, with mitochondria playing a central role in the complex processes. Proper mitochondrial function requires a plethora of different proteins, which are recruited into the organelle through well-defined inner membrane protein translocation machinery (1-3). The presequence translocase or the TIM23 complex accounts for import of approximately 60% of the total mitochondrial proteome and hence is critical for mitochondria biogenesis (4). In yeast, the subunit organization and functional annotations of the machinery are well established and show the presence of a single translocase performing the matrixdirected protein translocation. The yeast presequence translocase consists of a core channel composed of Tim23 along with Tim17. Both Tim23 and Tim17 are essential and form the channel component for the entry of the polypeptide chain. Nonessential accessory proteins, such as Tim21 and Pam17, are involved in conserved interactions with the core components and are important in the maintenance of the overall organization of the machinery. The TIM23 core channel is involved in a cooperative interaction with the matrix-directed import motor (composed of mtHsp70, Tim44, Mge1, and the Pam18-Pam16 subcomplex) in driving the import process (1, 2, 5-9). Tim23 and Tim17 form the central channel and along with Tim50 are involved in presorting the incoming polypeptide chains (1, 2, 4...

Mitochondrial protein translocases for survival and wellbeing

“…Both proteins stimulate the ATP‐ase activity of mtHsp70/Mortalin and interact with the TIM23 complex [60,62]. Although both proteins are ubiquitously expressed in human tissues, they are not fully interchangeable functionally [59]. MCJ, but not DNAJC19, was able to restore the viability of Pam18‐depleted yeast cells [62].…”

Functional Diversity of Human Mitochondrial J-proteins Is Independent of Their Association with the Inner Membrane Presequence Translocase

“…Magmas forms a part of the human import motor (23) and regulates the import process by inhibiting the human mtHsp70 (also known as Mortalin) ATPase stimulatory activity of its J-protein counterpart, DnaJC19 (JC19) (24). Recent reports suggest the presence of another member of DnaJ protein family DnaJC15 (JC15) in human mitochondria that may be potentially involved in the mitochondrial protein import process (25,26). This is in contrast with the yeast system where the overall import process is governed by a single J-protein, Pam18 (27,28).…”

“…First, we assessed the effect of J-proteins' down-regulation on the maintenance of mitochondria biogenesis by analyzing two parameters, namely, the amount of functional mitochondria and overall mitochondrial content (26). We found that down-regulation of Magmas and JC19 caused significant reduction in amount of functional mito- Because Magmas, JC19, and JC15 form a part of the mitochondrial protein transport complex, we assessed whether reduced mitochondria biogenesis is due to improper translocation of proteins into the organelle.…”

Coordinating Mitochondrial Biology Through the Stress-Responsive Regulation of Mitochondrial Proteases

“…In mammals, the IM TIM23 protein import complex exists in two distinct subunit compositions defined by the presence of a Tim17A or Tim17B subunit. Currently, functional differences between these two Tim17 homologs are poorly defined, although some experiments suggest that they differ in their ability to import non-traditional mitochondrial substrates (Sinha et al, 2014). In response to ISR-dependent translational attenuation, Tim17A, but not Tim17B, is rapidly degraded by the YME1L quality control protease (Rainbolt et al, 2013).…”