ABC transporters associated with cytochrome c biogenesis.

It is generally agreed that cytochrome c biogenesis requires that the apocytochrome and heme be transported separately to their site of function and assembly. In bacteria, this is outside the cytoplasmic membrane, whereby the apocytochromes c use sec-dependent signals for their translocation. Two different hypotheses have recently emerged as to how heme is exported: one involves an helABCD-encoded ATP binding cassette (ABC) transporter complex and the second does not. The second hypothesis concludes that an (HelAB)2 heterodimeric ABC transporter does not transport heme but some other substrate for cytochrome c biogenesis. The evidence supporting each of these two hypotheses and the role of this ABC transporter is discussed.