Disease relevance of TIM44

• To understand how this motor is anchored to the inner membrane, we have overexpressed Tim44 in Escherichia coli and studied the properties of the pure protein and its interaction with model lipid membranes [1].
• We investigated the role of Tim44 in the biogenesis of the authentic mitochondrial protein Yfh1p, the yeast homolog of mammalian frataxin, which was recently implicated in Friedreich ataxia [2].

High impact information on TIM44

• Precursor proteins carrying amino-terminal targeting signals are translocated into the matrix by the integral inner membrane proteins Tim23 and Tim17 in cooperation with Tim44 and mitochondrial Hsp70 [3].
• Tim44 is associated with this channel at the matrix side, and Tim44 recruits mitochondrial Hsp70 and its co-chaperone Mgel, which drive protein translocation into the matrix using ATP as an energy source [4].
• This molecular chaperone of the mitochondrial matrix is recruited to the protein import machinery by MIM44, a component associated with the inner membrane of the mitochondria [5].
• MIM44 and mt-Hsp 70 interact in a sequential manner with incoming segments of unfolded preproteins and thereby facilitate stepwise vectorial translocation of proteins across the mitochondrial membranes [5].
• The novel J-protein (encoded by PAM18/YLR008c/TIM14) is required for the interaction of mtHsp70 with Tim44 and protein translocation into the matrix [6].

Biological context of TIM44

• The essential gene TIM44 encodes a subunit of the inner mitochondrial membrane preprotein translocase that forms a complex with the matrix heat-shock protein Hsp70 [7].
• The membrane potential (delta psi)- and the mtHsp70/MIM44-dependent import machinery cooperate in the transfer of the presequence across the inner membrane [8].
• Hydrolysis of ATP by mt-Hsp70 occurs in complex with Tim44 [9].
• MPI1 encodes a hydrophilic 48.8 kDa protein that is essential for cell viability [10].
• One mutant mtHsp70 (Ssc1-2p) efficiently bound preproteins, but did not show a detectable complex formation with Tim44; the mitochondria imported loosely folded preproteins with wild-type kinetics, yet were impaired in unfolding of preproteins [11].

Anatomical context of TIM44

• Upon depletion of Tim44, mitochondria also lost their ability to import proteins but maintained normal numbers of import channels [12].
• Preprotein import into mitochondria is mediated by translocases located in the outer and inner membranes (Tom and Tim) and a matrix Hsp70-Tim44 driving system [13].
• The essential yeast gene MPI1 encodes a mitochondrial membrane protein that is possibly involved in protein import into the organelle (A. C. Maarse, J. Blom, L. A. Grivell, and M. Meijer, EMBO J. 11:3619-3628, 1992) [14].

Associations of TIM44 with chemical compounds

• On solubilizing mitochondria, ISP45 was rapidly converted to a 40 kDa proteolytic fragment unless mitochondria were first denatured with trichloroacetic acid [15].
• Both chaperones reversibly dissociate from Isp45 upon addition of ATP or adenosine 5'-[gamma-thio]triphosphate, suggesting that dissociation requires the binding of ATP [16].
• Probing the membrane topology of a subunit of the mitochondrial protein translocase, Tim44, with biotin maleimide [17].
• Our data indicate that a single cysteine residue, Cys-369, located in the C-terminal domain of the yeast Tim44 is exposed to the mitochondrial intermembrane space [17].
• Here, we have investigated the topology of Tim44 by probing mitochondria with membrane impermeable 3-(N-maleimidopropionyl)biocytin (MPB) followed by the specific immunoprecipitation of modified proteins [17].

Physical interactions of TIM44

• We conclude that Mge1p exerts an essential function in import and folding of proteins by controlling the nucleotide-dependent binding of mt-Hsp70 to substrate proteins and the association of mt-Hsp70 with MIM44 [18].

Other interactions of TIM44

• A screening for yeast mutants impaired in mitochondrial protein import led to the identification of two genes (MPII and MPI2) encoding the essential components MIM44 and MIM17 of the inner membrane import machinery [19].
• Outer and inner mitochondrial membrane receptors shown to participate in translocation of mitochondrial pre-proteins (MOM19 and MPI1/MIM44, respectively) are also important for import of the tRNA, in vitro as well as in vivo [20].
• Three essential subunits of the motor are known: mitochondrial Hsp70 (mtHsp70); the peripheral membrane protein Tim44; and the nucleotide exchange factor Mge1 [6].
• However, even under these conditions, no interaction of Ssq1 with the two other mitochondrial Hsp70-cochaperones, Tim44 and Mdj1, was observed [21].
• Mitochondrial protein import: recognition of internal import signals of BCS1 by the TOM complex [22].

Analytical, diagnostic and therapeutic context of TIM44

• Limited proteolysis and analytical ultracentrifugation indicate that Tim44 is an elongated monomer with a stably folded C-terminal domain [1].

References