ABSTRACT
c-type cytochromes are located partially or completely in the periplasm of gram-negative bacteria, and the heme prosthetic group is covalently bound to the protein. The cytochrome c maturation (Ccm) multiprotein system is required for transport of heme to the periplasm and its covalent linkage to the peptide. Other cytochromes and hemoglobins contain a noncovalently bound heme and do not require accessory proteins for assembly. Here we show that Bradyrhizobium japonicum cytochrome c
550 polypeptide accumulation in Escherichia coli was heme dependent, with very low levels found in heme-deficient cells. However, apoproteins of the periplasmic E. coli cytochrome b
562 or the cytosolic Vitreoscilla hemoglobin (Vhb) accumulated independently of the heme status. Mutation of the heme-binding cysteines of cytochrome c
550 or the absence of Ccm also resulted in a low apoprotein level. These levels were restored in a degP mutant strain, showing that apocytochrome c
550 is degraded by the periplasmic protease DegP. Introduction of the cytochrome c heme-binding motif CXXCH into cytochrome b
562 (c-b
562) resulted in a c-type cytochrome covalently bound to heme in a Ccm-dependent manner. This variant polypeptide was stable in heme-deficient cells but was degraded by DegP in the absence of Ccm. Furthermore, a Vhb variant containing a periplasmic signal peptide and a CXXCH motif did not form a c-type cytochrome, but accumulation was Ccm dependent nonetheless. The data show that the cytochrome c heme-binding motif is an instability element and that stabilization by Ccm does not require ligation of the heme moiety to the protein.
Proposed structure for the noncovalently associated heme prosthetic group of dissimilatory nitrite reductases. Identification of substituents.
