Biogenesis of Tim proteins of the mitochondrial carrier import pathway: Differential targeting mechanisms and crossing over with the main import pathway

Two major routes of preprotein targeting into mitochondria are known. Prepr oteins carrying amino-terminal signals mainly use Tom20, the general import pore (GIP) complex and the Tim23-Tim17 complex. Preproteins with internal signals such as inner membrane carriers use Tom70, the GIP complex, and the special Tim pathway, involving small Tims of the intermembrane space and T im22-Tim54 of the inner membrane. Little is known about the biogenesis and assembly of the Tim proteins of this carrier pathway. We report that import of the preprotein of Tim22 requires Tom20, although it uses the carrier Ti m route. In contrast, the preprotein of Tim54 mainly uses Tom70, yet it fol lows the Tim23-Tim17 pathway. The positively charged amino-terminal region of Tim54 is required for membrane translocation but not for targeting to To m70. Ln addition, we identify two novel homologues of the small Tim protein s and show that targeting of the small Tims follows a third new route where surface receptors are dispensable, yet Tom5 of the GIP complex is crucial. We conclude that the biogenesis of Tim proteins of the carrier pathway can not be described by either one of the two major import routes, but involves new types of import pathways composed of various features of the hitherto known routes, including crossing over at the level of the GIP.