J Mol Biol. 2014 Mar 29. pii: S0022-2836(14)00157-0. doi: 10.1016/j.jmb.2014.03.012. [Epub ahead of print]
Why have Small Multidrug-Resistance proteins not evolved into fused, internally duplicated structures?
Author information
- 1Center for Biomembrane Research, Department of Biochemistry and Biophysics, Stockholm University, SE-10691 Stockholm, Sweden.
- 2Center for Biomembrane Research, Department of Biochemistry and Biophysics, Stockholm University, SE-10691 Stockholm, Sweden; Science for Life Laboratory Stockholm University, SE-17177 Solna, Sweden. Electronic address: gunnar@dbb.su.se.
Abstract
The increasing number of solved membrane protein structures has led to the recognition of a common feature in a large fraction of the small molecule transporters: inverted repeat structures, formed by two fused homologous membrane domains with opposite orientation in the membrane. An evolutionary pathway has been posited in which the ancestral state is a single gene encoding a dual-topology membrane protein capable of forming antiparallel homodimers. A gene-duplication event enables the evolution of two oppositely orientated proteins that form antiparallel heterodimers. Finally, fusion of the two genes generates an internally duplicated transporter with two oppositely orientated membrane domains. Strikingly, however, in the Small Multidrug-Resistance (SMR) family of transporters, no fused, internally duplicated proteins have been found to date. Here, we have analyzed fused versions of the dual-topology transporter EmrE, a member of the SMR family, by BN-PAGE and in vivo activity measurements. We find that fused constructs give rise to both intramolecular inverted repeat structures and competing intermolecular dimers of varying activity. The formation of several intra- and intermolecularly paired species indicates that a gene fusion event may lower the overall amount of active protein, possibly explaining the apparent absence of fused SMR proteins in nature.
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