Cell Surface Structures of Archaea

Prokaryotes possess various kinds of cell surface organelles serving versatile biological roles depending on the environmental niche of the organism.The formation of these structures involves fascinating machineries, as not only do the protein components need to travel across the cytoplasmic membrane like all secreted proteins, but they also need to do so in a precisely coordinated manner for proper assembly.Most commonly found on the surface of bacteria are flagella used for swimming (47); the type III secretion injectisome (needle structure) (21), which is used to deliver effector molecules from pathogenic organisms into host cells; and a wide variety of thinner organelles that fall under the broad designation of pili (13,33,58,64,69,78).Different classes of these structures (type I pili, type IV pili, sex pili, etc.) which differ significantly in their structure, assembly, and function have been identified.Their many roles include adhesion, twitching (or surface) motility, and delivery of DNA and toxins, as well as functioning as electrically conductive "nanowires."Other, less commonly studied appendages have also been reported, such as spinae (9).Archaea, representing the third domain of life, have been isolated from the most extreme environments known to harbor life (15).They have been cultivated from environments that mark the upper limits of life in regard to temperature, pH, and salt concentration.Furthermore, molecular techniques have indicated that archaea are common in a variety of less extreme niches, such as ocean waters, freshwater sediments, and soil (20,23,66).Evidence points to a broader and more significant role in the ecosystem for archaea than previously believed (12,14,49).Both directed studies of surface structures in archaea, as well as electron microscopic observations of a variety of diverse archaea, have indicated a number of different types of surface structures.Some, like the archaeal flagella and pili, appear at first glance to be similar to their bacterial counterparts but possess significant differences and archaeal twists.Other structures, such as cannulae and hami, appear to be unique to the archaeal domain.Still other structures, such as the bindosome, are predicted to exist from genetic and biochemical data although direct electron microscopic evidence is, as yet, lacking.Studies on archaeal extracellular structures have enriched our