9.B.62 The Copper Resistance (CopD) Family
The CopD family consists of many homologues with 8 putative TMSs and sizes of about 300 residues from Gram-negative and Gram-positive bacteria. Some homologues from Gram-positive bacteria are larger (Listeria monocytogenes, 541 aas; Corynebacterium efficiens, 356 aas; Streptomyces coelicolor, 680 aas). Some of these proteins are multidomain proteins with extra soluble or membrane integrated domains of unknown function.
The E. coli homologue, PcoD, present in an operon (pcoABCDRS), has been characterized from plasmid pRJ1004 (Brown et al., 1995). It is homologous to the CopD protein of Pseudomonas syringae pv. tomato. The proteins in this operon appear to catalyze copper efflux in the logarithmic growth phase but allow accumulation in stationary phase (Brown et al., 1995). Other constituents present in the outer membrane (PcoB) and in the periplasm (PcoA and C) may be involved (Lee et al., 2002; Silver and Ji, 1994). Transcription of pco/cop operons may be controlled by two-component systems such as the PcoS/PcoR sensor kinase/response regulator system. The mechanism of copper resistance is not known but may involve either (1) copper efflux, (2) copper uptake plus periplasmic copper sequestration by CopA and CopC, or (3) copper uptake by a two component CopD-CopC system coupled to an unknown resistance mechanism.