I am reading journal papers about the subcellular localisation of the insulin receptor (IR) in neurons.
I have read a paper stating that IR is highly enriched at synapses, localising to both the presynaptic axon terminal and the postsynaptic density compartments.
The above statement saying that IR localises to presynaptic axon terminals comes from immunofluorescence images of cultured neurons where IR colocalises with synaptophysin puncta.
In contrast, another paper showed via subcellular fractionation of the rat brain that IR is enriched in the post-synaptic density (PSD) fraction (through analysing the IR levels in the PSD fraction via Western blotting).
I think this is stronger evidence compared to immunocytochemistry.
To say that a protein is a component of the presynaptic terminal is colocalisation with a presynaptic marker (e.g. synaptophysin) sufficient evidence? Or should there be other forms of evidence to back this statement up?