What is the nature of the mitochondrial permeability transition pore and what is it not?

Since about 60 years a phenomenon now called permeability transition is known in mitochondria. It involves a large pore in the inner mitochondrial membrane, the permeability transition pore (PTP) whose molecular structure is still unknown. Year after year, new hypotheses have been developed how this pore could look like and which proteins cold be structural elements.