TIM23 is a core component of the mitochondrial protein import machinery that resides in the inner mitochondrial membrane. As part of the TIM23 complex (translocase of the inner mitochondrial membrane 23), TIM23 forms a channel through which precursor proteins are imported from the cytosol into the mitochondrial matrix or inserted into the inner membrane[1]. This essential translocase mediates the import of over 1,000 nuclear-encoded proteins that are synthesized in the cytosol but function within mitochondria, including critical components of the electron transport chain, ATP synthase, and mitochondrial ribosomes.
Given the central role of mitochondria in neuronal function—particularly in high-energy demanding dopaminergic neurons and other neuronal subtypes—TIM23-mediated protein import is fundamental to neuronal survival. Dysregulation of mitochondrial protein import has emerged as a significant contributor to neurodegenerative disease pathogenesis, particularly in Parkinson's disease where complex I deficiency is a hallmark feature and in Alzheimer's disease where mitochondrial dysfunction is an early event[2][3].
TIM23 is a ~16 kDa integral membrane protein with distinctive structural features:
The protein adopts a overall "L"-shaped architecture with:
TIM23 shares structural homology with other Tim17/Tim22/Tim23 family members:
TIM23 operates as part of a larger complex:
| Component | Function |
|---|---|
| TIM23 | Channel-forming subunit, lateral gate |
| Tim17 | IMS domain, substrate access |
| Tim21 | Assembly, interaction with OXPHOS |
| Tim50 | Transfer to Tim23 from TOM |
| Tim44 (matrix) | Motor recruitment |
The core TIM23 complex contains TIM23 and Tim17 in a 1:1 ratio, forming the minimal translocase channel.
The TIM23 channel operates through conformational changes[4]:
The primary function of TIM23 is importing precursor proteins synthesized in the cytosol[5]:
Import Pathway:
Substrate Types:
TIM23 import requires energy:
The membrane potential (ΔΨm) is essential—depolarization impairs import.
TIM23 functions in concert with the TOM complex (translocase of outer membrane)[6]:
TIM23 imports numerous essential proteins:
Electron Transport Chain subunits:
Other essential proteins:
TIM23 activity is regulated[7]:
Parkinson's disease involves prominent mitochondrial dysfunction[8]:
Complex I Deficiency:
TIM23 Contributions:
The PINK1/Parkin mitophagy pathway intersects with TIM23 function[9]:
Pathological Cascade:
Dopaminergic neurons in the substantia nigra pars compacta show particular vulnerability:
High Energy Demands:
Susceptibility Factors:
Targeting mitochondrial protein import in PD:
Stabilizing Import:
Alzheimer's disease involves significant mitochondrial dysfunction[3:1]:
Early Changes:
Contributing Factors:
Aβ localizes to mitochondria and affects function[10]:
Tau pathology intersects with mitochondrial function:
The brain relies heavily on oxidative phosphorylation[11]:
Amyotrophic lateral sclerosis involves mitochondrial dysfunction[12]:
Motor Neuron Vulnerability:
TIM23 Contributions:
Mitochondrial function declines with age[13]:
Proper import ensures protein quality:
TIM23 and imported proteins are turned over:
Mitochondrial protectants:
Targeting TIM23:
Exercise enhances mitochondrial function:
TIM23 activity or assembly may serve as biomarker:
TIM23 is evolutionarily conserved[14]:
TIM23 is an essential component of the mitochondrial protein import machinery, forming the channel through which nuclear-encoded proteins are imported into the mitochondrial inner membrane or matrix. This function is critical for maintaining the electron transport chain, ATP production, and overall mitochondrial function. In neurodegenerative diseases including Parkinson's and Alzheimer's, mitochondrial dysfunction is a hallmark, and impaired protein import through TIM23 contributes to this pathology. The particular vulnerability of dopaminergic neurons in PD—due to their high energy demands—makes proper TIM23 function especially important. Therapeutic approaches targeting mitochondrial protein import, including stabilizing TIM23 function and preserving the mitochondrial membrane potential, represent promising strategies for treating neurodegenerative conditions.
Chacinska A, et al. Structure of the TIM23 complex. Cell. 2002. ↩︎
Ryan BJ, et al. Mitochondrial dysfunction in Parkinson's disease. Cell Death and Disease. 2014. ↩︎
Swerdlow RH, et al. Mitochondrial dysfunction in Alzheimer's disease. Journal of Alzheimer's Disease. 2010. ↩︎ ↩︎
Truscott KN, et al. Molecular mechanism of TIM23-mediated import. EMBO Journal. 2003. ↩︎
Neupert W, Herrmann JM. Translocation of proteins into mitochondria. Annual Review of Biochemistry. 2007. ↩︎
Yamamoto H, et al. The TOM complex: protein import machinery. Journal of Biochemistry. 2011. ↩︎
Rehling P, et al. Assembly of TIM23 complex. Journal of Cell Biology. 2003. ↩︎
Schapira AH, et al. Complex I deficiency in Parkinson's disease. Lancet. 1998. ↩︎
Narendra DP, et al. PINK1 and Parkin in mitophagy. Journal of Cell Biology. 2010. ↩︎
Mattson MP, et al. Amyloid-beta and mitochondrial dysfunction. Journal of Neuroscience Research. 2002. ↩︎
Attwell D, Laughlin SB. Brain energy metabolism. Journal of Cerebral Blood Flow and Metabolism. 2001. ↩︎
Coombes ON, et al. Mitochondrial dysfunction in ALS. Brain Research Bulletin. 2012. ↩︎
Kann O, et al. Mitochondrial function and aging. Journal of Cerebral Blood Flow and Metabolism. 2011. ↩︎
Gray MW, et al. Evolution of mitochondrial protein import. Biological Reviews. 2012. ↩︎