SUMMARY PARAGRAPH for SSC1
About mitochondrial import
While the mitochondrial genome encodes a handful of proteins, most of the hundreds of proteins that reside in the mitochondrion are encoded by nuclear genes, translated in the cytoplasm, and imported into mitochondria via a series of complex molecular machines (see 7, 8 for review). Many of the proteins imported into mitochondria are involved in respiration, which is not an essential process: S. cerevisiae is able to carry out either fermentative growth on carbon sources such as glucose, or respiratory growth on nonfermentable carbon sources such as glycerol and ethanol. However, since maintenance of the mitochondrial compartment is essential to life, mutations that completely disrupt mitochondrial import are lethal.
About the TIM23 complex
The Translocase of the Inner Mitochondrial membrane (TIM23 complex) receives proteins from the Translocase of the Outer Mitochondrial membrane (TOM complex) and either directs them into the mitochondrial matrix or facilitates their integration into the mitochondrial inner membrane (reviewed in 9, 8, 10). The membrane-embedded core of the complex is composed of three essential proteins: Tim23p, Tim17p, and Tim50p. Tim23p and Tim17p, which share sequence similarity, comprise the twin-pore structure through which precursor proteins translocate. Tim23p alone has the ability to form a voltage-sensitive channel (11), but Tim17p is required in vivo for maintenance of the twin-pore architecture and for normal function of the pore (12). Tim17p also has a role in sorting incoming proteins to the mitochondrial matrix or the inner membrane (13). Tim50p interacts with precursor proteins and with Tim23p to guide precursors from the TOM complex to the TIM23 complex (14, 15). Two additional non-essential components, Tim21p and Pam17p, interact with the core of the TIM23 complex and may modulate its activity (13, 16, 17).
Proteins destined for the mitochondrial matrix require the action of a sub-complex of the TIM23 complex, known as the import motor or presequence translocase-associated motor (PAM) complex. Its catalytic component is Ssc1p, a member of the heat shock 70 protein family commonly referred to as mtHsp70, which undergoes cycles of binding and release of the precursor, hydrolyzing ATP and changing conformation in the process. The nucleotide release factor Mge1p promotes this cycle by facilitating the dissociation of ADP from Ssc1p (18, 19). Other components include Tim44p, an essential subunit that mediates the association of the core TIM23 complex with the PAM complex (20, 17); Pam18p (Tim14p), a J-protein cochaperone that stimulates the ATPase activity of Ssc1p; and Pam16p (Tim16p), a J-like protein that binds to Pam18p and regulates its activity (21). Pam17p mediates the association between Pam16p and Pam18p (22). Once imported proteins reach the mitochondrial matrix, their correct folding is facilitated by a soluble complex consisting of Ssc1p and its cochaperones Mdj1p and Mge1p (23).
A subset of proteins destined for insertion into the mitochondrial inner membrane is translocated via the TIM23 complex but then inserted laterally into the inner membrane rather than entering the mitochondrial matrix. This mechanism is currently not understood in detail. The TIM23 complex adopts different conformations during the two kinds of import, but it is unclear whether this inner membrane import is accomplished by the core complex alone (Tim23p, Tim17p, and Tim50p), or by the entire TIM23 complex including the import motor subunits (9, 16).
SSC1 encodes an essential chaperone that is the key component of the import motor sub-complex that mediates the transit of precursor proteins through the TIM23 complex (1, 24, 25). Ssc1p uses energy derived from ATP hydrolysis to facilitate protein translocation, with the help of its cochaperones Pam18p and Mge1p (18, 19). It also forms a soluble complex in the mitochondrial matrix, consisting of Ssc1p with its cochaperones Mge1p and Mdj1p, that re-folds newly imported proteins (26). Additionally, Ssc1p binds to Ens2p, a mitochondrially-encoded protein whose gene is present in some S. cerevisiae strains but not others, to act as a regulatory subunit of the site-specific endonuclease Endo.SceI (2). Like all other Hsp70 proteins, Ssc1p contains an N-terminal ATPase domain; in the absence of nucleotide binding Ssc1p is prone to form insoluble self-aggregates, an event which can be prevented by the chaperone protein Zim17p (27).
SSC1, SSQ1, and ECM10 encode chaperone proteins of the HSP70 family that localize to the mitochondria (28 and reviewed in 29). In addition to these three mitochondrial HSP70s, S. cerevisiae cells also synthesize nine cytosolic HSPs (encoded by SSA1, SSA2, SSA3, SSA4, SSB1, SSB2, SSE1, SSE2, SSZ1) and two that are found in the ER (KAR2, LHS1). HSP70 is a large family of proteins that has been evolutionarily conserved from bacteria (DnaK) to humans (HSP72/73). HSP70 proteins were originally classified based upon their induction by heat shock and their size of ~70kDa. The main function of these proteins is to serve as molecular chaperones, binding unfolded peptides to assist in proper folding and prevent aggregation/misfolding (reviewed in 30 and 31). HSP70s are also involved in disassembling aggregates of misfolded proteins, translocating select proteins into the mitochondria and ER, and degrading aberrant proteins (reviewed in 32, 31, and 30).
Last updated: 2006-02-09