A water-soluble form of porin from the mitochondrial outer membrane of Neurospora crassa

Mitochondrial porin, the outer membrane pore-forming protein, was
isolated in the presence of detergents and converted into a water-
soluble form. This water-soluble porin existed under nondenaturing
conditions as a mixture of dimers and oligomers. The proportion of
dimers increased with decreasing porin concentration during
conversion. Water-soluble porin inserted spontaneously into
artificial bilayers as did detergent-solubilized porin. Whereas the
latter form had no specific requirements for the lipid composition
of the bilayer, water- soluble porin inserted only into membranes
containing a sterol, and only in the presence of very low
concentrations of Triton X-100 (0.001% w/v) in the solution bathing
the bilayer. The channels formed by water- soluble porin were
indistinguishable from those formed by detergent- purified porin
with respect to specific conductance and voltage dependence of
conductance. Water-soluble porin bound tightly in a saturable
fashion to isolated mitochondria. The bound form was readily
accessible to added protease, indicating its presence on the
mitochondrial surface. The number of binding sites was in the range
of 5-10 pmol/mg of mitochondrial protein. Water-soluble porin
apparently binds to a site on the assembly pathway of the porin
precursor, since mitochondria whose binding sites were saturated
with the water-soluble form did not import porin precursor
synthesized in a cell-free system.