EVOLUTIONARY LINK BETWEEN PROKARYOTIC AND EUKARYOTIC K+ CHANNELS

Considering the importance of K+ channels in controlling the crucial K + gradient across the plasma membranes of all living cells, it comes a s no surprise that, besides being present in every eukaryotic cell, th ese integral membrane proteins have recently also been identified in p rokaryotes. Today, approximately a dozen successfully completed and ma ny more ongoing sequencing projects permit a search for genes related to K+ channels in the genomes of both eubacteria and archaea, The codi ng regions of homologues show a remarkable variety in primary structur e. They predict membrane proteins with one, two, three and six hydroph obic segments surrounding a putative K+-selective pore (H5) and the pr esence or absence of a cytosolic putative NAD(+)-binding domain (PNBD) that probably senses the reducing power of the cell. The analysis of kinships on the basis of phylogenetic algorithms identifies sequences closely related to eukaryotic voltage-dependent K+ channels, but also defines members of a primordial class of prokaryotic K+ channel (conta ining the 2TMS/PNBD motif). Considering the unique mechanisms that may account for the assembly of modern proteins from different ancestral genes, and with more primary sequence data soon to appear, a scheme fo r the evolutionary origin of K+ channels comes within reach.