PORINS FROM PLANTS - MOLECULAR-CLONING AND FUNCTIONAL-CHARACTERIZATION OF 2 NEW MEMBERS OF THE PORIN FAMILY

Porins are voltage-gated diffusion pores found in all eukaryotic kingd oms. Here we describe, for the first time, the identification and char acterization of two cDNAs encoding porins from plants. Peptide sequenc es 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 co mparison against the Swiss-Prot sequence data base revealed a homology of about 25% to mitochondrial porins from fungi and human. Computer-a ided predictions of the secondary structure of the plant porins reveal ed the presence of 16 antiparallel beta-strands that are also found in mitochondrial porins. Porins from non-green plastids and from the out er mitochondrial membrane were reconstituted into planar lipid bilayer s. The proteins showed high pore-forming activities and similar single -channel conductances. In vitro translated porin was preferentially im ported only into non-green plastids but not into chloroplasts. To our knowledge, this is the first example of selective import of a plastidi al protein into different types of plastids. This finding is in line w ith the observation that an immunoreactive 30-kDa band was only found in non-green plastids and mitochondria but not in chloroplasts, We con clude that mitochondria and non-green plastids possess homologous pori n proteins, whereas chloroplasts are characterized by a different type of porin.