SNARE proteins are found in the membranes of all of the following compartments EXCEPTShow answer
(A) Mitochondria
(B) Golgi complex
(C) Early endosome
(D) Endoplasmic reticulum
(E) Synaptic plasma membrane
This one is about such important process as membranes fusion. SNARE are twin membrane proteins (referred to as v-SNARE [vesicle] and t-SNARE [target]), that act as anchors when two vesicles decide to fuse into one. This process is so important because it participate in molecular transport in the cell. Membrane and secretory proteins pass through several membrane-bounded organelles. Simple pathway of protein can be described as (you will find much more info in Alberts' MBoC) nucleus –> endoplasmic reticulum (ER) –> Golgi apparatus (or complex) -> plasma membrane OR endosome (pro-lysosome) OR exterior. Origin of vesicles plays its role in sinaptic signal transduction. The reason why there are no SNARE proteins in the mitochondria membrane is described while talking about mitochondria itself.
But before speak about mitochondria, lets remember why vesicles are essential for proteins movement. The main idea – as I see – is so-called co-translational translocation of proteins. In plain english it means, that proteins are translated by ribosomes only when they are bound to ER membrane. Thus proteins are translated into the ER, because if they were translated into cytoplasm, they could fold unproperly (by the way, about 80% of all membrane proteins fold unproperly even inside ER). But mitochondrial proteins that are imported (because mitochondria have its own gene translation machinery and so produce many essential proteins by itself) are translocated post-translationally, after ribosomes actually detach from polypeptide. After that so-called precursor proteins are imported into mitochondrial matrix through TOM and TIM complexes led by signal polypeptide chain (note that there are two membranes! that complexes act like gateway, really fascinating structure) just like membrane proteins being synthesized by ribosome are dragged to ER by Signal Recognition Particle (SRP) bound to protein's signal chain.
Precursor protein does not fold in the cytoplasm because of interactions with other proteins, some of which are general chaperones like hsp70 family.
There is a lot of things to tell about mitochondrial proteins translocation (imagine that some proteins, produced by the cell, are membrane mitochondrial proteins and their translocation requires several different signal polypeptide chains). But the point is that there is no possibility for membrane fusion between vesicles and mitochondrial bi-membranes so no SNARE needed for protein translocation. And the answer is A.
But before speak about mitochondria, lets remember why vesicles are essential for proteins movement. The main idea – as I see – is so-called co-translational translocation of proteins. In plain english it means, that proteins are translated by ribosomes only when they are bound to ER membrane. Thus proteins are translated into the ER, because if they were translated into cytoplasm, they could fold unproperly (by the way, about 80% of all membrane proteins fold unproperly even inside ER). But mitochondrial proteins that are imported (because mitochondria have its own gene translation machinery and so produce many essential proteins by itself) are translocated post-translationally, after ribosomes actually detach from polypeptide. After that so-called precursor proteins are imported into mitochondrial matrix through TOM and TIM complexes led by signal polypeptide chain (note that there are two membranes! that complexes act like gateway, really fascinating structure) just like membrane proteins being synthesized by ribosome are dragged to ER by Signal Recognition Particle (SRP) bound to protein's signal chain.
Precursor protein does not fold in the cytoplasm because of interactions with other proteins, some of which are general chaperones like hsp70 family.
There is a lot of things to tell about mitochondrial proteins translocation (imagine that some proteins, produced by the cell, are membrane mitochondrial proteins and their translocation requires several different signal polypeptide chains). But the point is that there is no possibility for membrane fusion between vesicles and mitochondrial bi-membranes so no SNARE needed for protein translocation. And the answer is A.
