PREPROTEINS OF CHLOROPLAST ENVELOPE INNER MEMBRANE CONTAIN TARGETING INFORMATION FOR RECEPTOR-DEPENDENT IMPORT INTO FUNGAL MITOCHONDRIA

The amino-terminal transit sequences of two preproteins destined for t he chloroplast inner envelope membrane show similarities to mitochondr ial presequences in the prevalence of positive charges and the potenti al formation of an amphipathic alpha-helix. We studied if these prepro teins could be imported into mitochondria and found a low, yet signifi cant import into isolated plant mitochondria. The plant mitochondria w ere previously shown not to import precursors of chloroplast stromal o r thylakoidal proteins. To analyze the specificity of import into mito chondria we used the established import systems of fungal mitochondria . The envelope preproteins were efficiently imported into Saccharomyce s cerevisiae or Neurospora crassa mitochondria. Their import showed th e characteristics of specific mitochondrial protein uptake, including a requirement for the main receptor MOM19 (mitochondrial outer membran e protein of 19 kDa) and a membrane potential across the inner membran e, and depended on the presence of the chloroplast transit sequence. W e conclude that some chloroplast transit sequences contain sufficient information for specific interaction with mitochondrial import recepto rs (at least from fungal sources).