Lecture 14 - mitochondria and chloroplasts part 2 Flashcards
When does mitochondrial fusion occur?
in response to cell stress - increased need for coordinated functioning of mitochondria network
What does mitochondrial fusion require?
sequential fusion of both mitochondrial membranes, first outer membranes and then inner membranes
What are the steps to mitochondrial fusion?
- outer membrane tethering
- GTPase mitofusins - Mfn1 and Mfn2, link together in GTP dependent manner to form organelle tethering complex
- proper Mfn1/2 binding regulated by other mitochondrial outer membrane proteins Bak and Bax - outer membrane fusion
- formation of outer membrane lipid microdomains at sites of Mfn1/2 tethering- phospholipase D converts cardiolipin into phosphatidic acid - causes outer membrane curvature inward and promotes mfn1/2 mediated membrane fusion via GTP hydrolysis
- inner membrane cristae fusion
- mediated by OPA1 - contains intermembrane space facing GTPase domain
- OPA1 proteins on adjacent inner membranes interact in GTP dependent manner to promote membrane fusion vi GTP hydrolysis
- OPA1 binding is regulated by other mitochondrial inner membrane proteins such as prohibitin
What are Mfn1 and Mfn2?
- integral outer membrane proteins
- possess cytoplasmic facing GTPase domain and long, coiled coil protein protein interaction domain
- located on adjacent mitochondria - link together in GTP dependent manner to form organelle tethering complex
What is the function of prohibitin?
inner mitochondrial membrane protein that ensures that OPA1 mediated fusion occurs only between different inner membranes. prevents self fusion of cristae within same mitochondrion
How do mitochondrial proteins go to the mitochondria?
majority of mitochondrial proteins are nuclear-encoded, they synthesize on free ribosomes in the cytoplasm and target post-translationally to the organelle
How does cell ensure that nascent protein is properly targeted to mitochondrion and to current mitochondrial subcomp?
all nuclear encoded mitochondrial proteins possess unique targeting sequences - specific sequence of AA serves as zipcode
very complex because there are multiple targeting pathways
the targeting sequence varies depending on final location in mitochondrion (outer mem, inner mem, inner mem matrix)
What is the best understood targeting to mitochondria process?
to mitochondrial matrix proteins
How long is the AA sequence for the targeting sequence on mitochondrial matrix destined proteins?
20-50
Where is the matrix targeting sequence located on the mitochondrial matrix protein?
N-terminus
What does the matrix targeting sequence located on the mitochondrial matrix protein contain?
amphipathic alpha helix - enriched in positively charged residues on one side of helix and hydroxylated residues on other side
What is the function of the targeting sequence on the mitochondrial matrix protein?
targeting nascent matrix protein to cyto[plasmic surface of mitochondrion and subsequent translocation across outer and inner membranes
- the targeting sequence is cleaved following protein import into matrix
What are the steps to the matrix protein targeting and import?
- in cytosplasm precursor protein synthesized on free ribosomes and recognized by cytoplasmic molecular chaperones
- protein state maintained as partially unfolded import competent state
- nascent mitochondrial proteins enriched in vicinity of mitochondria surface due to diffusion and mRNA localization - at surface of mitochondrion, proteins matrix targeting sequence recognized by import receptor complex which consists primarily of - tom20 and tom 22, also consists of several accessory proteins that serve as scaffold
- precursor protein passed from import receptor to general import pore in outer membrane - consists of tom40 which forms a transmembrane channel that allows for protein translocation across the outer membrane
- precursor protein is transferred through general import pore and then through adjacent inner membrane channel which consists of tim17 tim23 and tim44, precusor protein translocation occurs across both membranes sequentially
- N-terminal matrix targeting sequence of precursor proteins exits inner membrane channel into matrix, it is cleaved by matrix processing protease, emerging precursor protein also recognized and bound by matrix Hsp70 which is located at the matrix face of inner membrane channel via binding to tim44. matrix hsp70 acts as molecular motor (ratchet) undergoes ATP dependent conformational changes that pulls protein into matrix and prevents back sliding of protein back into cytoplasm
- import driver partially by H+ electrochemical gradient across inner membrane H+ intermembrane space > H+ matrix - imported cleaved mature protein in matrix folds without further assistance into final active conformation
OR - final folding of imported cleaved protein in matrix requires additional matrix localized molecular chaperones and ATP hydrolysis
What is a mitochondrial RNA cloud?
mRNAs encoding mitochondrial proteins often enriched in cytoplasm surrounding mitochondria
results in mitochondrial protein synthesis (tln) taking place immediately adjacent to mitochondrial surface
What is the mitochondrial RNA cloud mediated by?
RNA-binding proteins located on mitochondrial outer surface
