Abstract
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.
Highlights
Porins are voltage-gated diffusion pores foundin all bacteria (Colombini, 1979; Benz,1985)(for a recent review see eukaryotic kingdoms.Herewe describe, for the first Benz (1994))
Little is known non-green plastids possess homologous porin proteins, yet aboutpore-forming proteins in the outer membroafnpelaswhereas chloroplasts are characterized by a different tids
The mitochondrial outer membrane shows a permeability for hydrophilic molecules up toa molecularmass of 4-5 kDa due to the presence of general diffusion pores called voltagedependent anion-selective channels or porins inanalogy to the pore-forming proteins in the outer memborfanGeram-negative higher than theexclusion limits of bacterial or mitochondrial porins (1-2 and 4-5 kDa, respectively)
Summary
MOLECULAR CLONING AND FUNCTIONAL CHARACTERIZATION OF TWO NEW MEMBERS OF THE PORIN FAMILY*. In uitro translated porin was preferentially imported only into non-greenplastids but not into chloroplasts.To our knowledge, this is the first example of selective import of a plastidial protein into different types of plastids. This finding is in line with the terminus of the protein and the large extramemblroaonpes are located at the cytosolic side of the membrane (De Pinto et al, 1991). Peptide sequences obtained from a 30kDa outer membrane protein allowed the isolation of cDNA clones from pea andmaize. The identityof the clones and therelationshipbetweenporins frommitochondria, chloroplasts, and non-green plastids are discussed
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