Transport of proteins into cryptomonads complex plastids

Complex plastids, found in many alga groups, are surrounded by three or fou r membranes. Therefore, proteins of the complex plastids, which are encoded in the cell nucleus, must cross three or four membranes during transport t o the plastid. To study this process we have developed a method for isolati ng transport-competent two membrane-bound plastids derived from the complex plastids of the cryptophyte Guillardia theta. This in vitro protein import system provides the first non-heterologous system for studying the import of proteins into four-membrane complex plastids. We use our import system a s well as canine microsomes to demonstrate in the case of cryptomonads how nuclear proteins pass the first nucleomorph-encoded proteins the third and fourth membrane and discuss the potential mechanisms for protein transport across the second membrane.