It is generally accepted that in the e-type cytochromes the covalently bound heme plays a primary role in the acquisition of the folded state. Here, we show that a stabilized site-directed variant of apo-cyt c(551) from Pseudomonas aeruginosa (Pa-apocyt F7A/W77F) retains native-like features in the presence of sodium sulfate even in the absence of heme. By time-resolved intrinsic fluorescence, we have evidence that Pa-apocyt F7A/W77F may acquire a compact, native-like conformation within microseconds. These results challenge current thinking about the role of the heme group in the folding of c-type cytochromes. (C) 2008 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.