MIP 28 forms channels in planar lipid bilayers

RESUMO

Fiber cells, which constitute most the lens tissue, have large amounts of a protein named the main intrinsic protein (MIP) in their plasma membrane, MIP seems to vary among species. On SDS-PAGE, MIP from bovine lens (MIP 26) migrates faster than MIP from chicken lens (MIP 28), which runs as a 28-KDa protein. Recently a number of laboratories have shown that MIP 26 forms channels in lipid bilayers. We have isolated membrane fractions highy enriched in MIP 28 from chicken lens and incorporated channel activity into planar bilayers from these membrane fractions before and after treatment with the detergent Triton X-100. Detergent treatment does not seem to affect channel properties. We have attempted to block channel activity with polyclonal antivodies against bovine and chicken MIP but failed to detect blockade using either detergent-free or detergent-treated membranes. Single channel size in symmetric solutions of 300 m§K2SO4(3-00-400pS) agrees well with published results one allows for corrections in ionic strenght. Preliminary experiments indicate that the incorporated channels display voltage dependence. The channel activity recorded from MIP 28-enriched membrane fractions ins qualitatively similar to that described for MIP 26 membrane fractions incorporated into bilayers. In contrast to previous reports, we do not find it necessary to add the membrane fractions to both sides of the bilayer to obtain channel incorporation. This may reflect the fact that MIP does not span two bilayers