Identification of porin-like polypeptide(s) in the boundary membrane of oilseed glyoxysomes

A 36-kDa polypeptide of unknown function was identified by us in the bounda ry membrane fraction of cucumber seedling glyoxysomes. Evidence is presente d in this study that this 36-kDa polypeptide is a glyoxysomal membrane pori n, A sequence of 24 amino acid residues derived from a CNBr-cleaved fragmen t of the 36-kDa polypeptide revealed 72% to 95% identities with sequences i n mitochondrial or non-green plastid porins of several different plant spec ies. Immunological evidence indicated that the 36-kDa land possibly a 34-kD a polypeptide) was a porin(s), Antiserum raised against a potato tuber mito chondrial porin recognized on immunoblots 34-kDa and 36-kDa polypeptides in detergent-solubilized membrane fractions of cucumber seedling glyoxysomes and mitochondria, and in similar glyoxysomal fractions of cotton, castor be an, and sunflower seedlings. The 36-kDa polypeptide seems to be a constitut ive component because it was detected also in membrane protein fractions de rived from cucumber leaf-type peroxisomes. Compelling evidence that one or both of these polypeptides were authentic glyoxysomal membrane porins was o btained from electron microscopic immunogold analyses. Antiporin IgGs recog nized antigen(s) in outer membranes of glyoxysomes and mitochondria, Taken together, the data indicate that membranes of cucumber land other oilseed) glyoxysomes, leaf-type peroxisomes, and mitochondria possess similar molecu lar mass porin polypeptide(s) (34 and 36 kDa) with overlapping immunologica l and amino acid sequence similarities.