Porins from plants. Molecular cloning and functional characterization of two new members of the porin family.

Porins are voltage-gated diffusion pores found in all eukaryotic kingdoms. Here we describe, for the first time, the identification and characterization of two cDNAs encoding porins from plants. Peptide sequences obtained from a 30-kDa protein of envelope membranes from pea root plastids allowed the isolation of two cDNA clones from pea and maize. On the protein level, both proteins are homologous by 58%. Sequence comparison against the Swiss-Prot sequence data base revealed a homology of about 25% to mitochondrial porins from fungi and human. Computer-aided predictions of the secondary structure of the plant porins revealed the presence of 16 antiparallel beta-strands that are also found in mitochondrial porins. Porins from non-green plastids and from the outer mitochondrial membrane were reconstituted into planar lipid bilayers. The proteins showed high pore-forming activities and similar single-channel conductances. In vitro translated porin was preferentially imported only into non-green plastids but not into chloroplasts. To our knowledge, this is the first example of selective import of a plastid protein into different types of plastids. This finding is in line with the observation that an immunoreactive 30-kDa band was only found in non-green plastids and mitochondria but not in chloroplasts. We conclude that mitochondria and non-green plastids possess homologous porin proteins, whereas chloroplasts are characterized by a different type of porin.