Evidence for zinc and cadmium binding in a CDF transporter lacking the cytoplasmic domain

Cation diffusion facilitators (CDFs) have been described as requiring the C-terminal cytoplasmic domain for their function. With the identification of smaller proteins lacking the cytoplasmic portion but displaying sequential characteristics of CDFs, this assumption should be reconsidered. Here we describe the results showing that the MmCDF3, a 23-kDa protein lacking a C-terminal domain, interacts selectively with zinc and cadmium. Isothermal titration calorimetry (ITC) binding results indicate that the truncated CDF may have an alternative means of acquiring ions from the cytoplasm in the form of an extended N-terminus, a feature common to putative cation efflux proteins of a similar size.