IMPORT OF MICROINJECTED PROTEINS BEARING THE SKL PEROXISOMAL TARGETING SEQUENCE INTO THE PEROXISOMES OF A HUMAN FIBROBLAST CELL-LINE - EVIDENCE THAT VIRTUALLY ALL PEROXISOMES ARE IMPORT-COMPETENT

Peroxisomes import virtually all of their membrane and matrix proteins post-translationally. It is presently unknown whether, in mammalian c ells, their exists a pool of mature peroxisomes which have received th eir complement of proteins and are import-incompetent. Previous work h as shown that fibroblasts are capable of importing microinjected perox isomal proteins into peroxisomes. This report describes the import of a hybrid peroxisomal protein into virtually all peroxisomes of the mic roinjected cell, The peroxisomal import was uniform in both short and long incubations, Pretreatment of the cells with cycloheximide did not affect the import of the peroxisomal protein, nor was there any diffe rence in the distribution of the imported protein. Sequential microinj ection experiments demonstrated that peroxisomes that had imported luc iferase were capable of importing another peroxisomal protein injected 24 hours later, These results suggest that, in fibroblasts, all perox isomes have associated protein-import machinery; this evidence does no t support the hypothesis that there exists a pool of import-incompeten t peroxisomes.