A universal system for the transport of redox proteins: early roots and latest developments

The transport of proteins binding redox cofactors across a biological membrane is complicated by the fact that insertion of the redox cofactor is often a cytoplasmic process. These cytoplasmically assembled redox proteins must thus be transported in partially or completely folded form. The need for a special transport system for redox proteins was first recognized for periplasmic hydrogenases in gram-negative bacteria. These enzymes, which catalyze the reaction H2 <--> 2H+ + 2e, are composed of a large and a small subunit. Only the small subunit has an unusually long signal sequence of 30-50 amino acid residues, characterized by a conserved motif (S/T)-R-R-x-F-L-K at the N-terminus. This sequence directs export of the large and small subunit complex to the periplasm. Sequencing of microbial genes and genomes has shown that signal sequences with this conserved motif, now referred to as twin-arginine leaders, occur ubiquitously and export different classes of redox proteins, containing iron sulfur clusters, molybdopterin cofactors, polynuclear copper sites or flavin adenine dinucleotide. Mutations in an Escherichia coli operon referred to as mtt (membrane targeting and translocation) or tat (twin arginine translocation) are pleiotropic, i.e. these prevent the expression of a variety of periplasmic oxido-reductases in functional form. The Mtt or Tat pathway is distinct from the well-known Sec pathway and occurs ubiquitously in prokaryotes. The fact that its component proteins share sequence homology with proteins of the delta pH pathway for protein transport associated with chloroplast thylakoid assembly, illustrates the universal nature of this novel protein translocation system.