Archaea are prokaryotic organisms that lack endomembrane structures. However, a number of hyperthermophilic members of the Kingdom Crenarchaea, including members of the Sulfolobus genus, encode homologs of the eukaryotic endosomal sorting system components Vps4 and ESCRT-III (endosomal sorting complex required for transport -III). We found that Sulfolobus ESCRT-III and Vps4 homologs underwent regulation of their expression during the cell cycle. The proteins interacted and we established the structural basis of this interaction. Furthermore, these proteins specifically localized to the mid-cell during cell division. Over-expression of a catalytically inactive mutant Vps4 in Sulfolobus resulted in the accumulation of enlarged cells, indicative of failed cell division. Thus, the archaeal ESCRT system plays a key role in cell division.Within the archaeal domain of life, there are two principal Kingdoms, the Crenarchaea and the Euryarchaea. Studies of microbial diversity have revealed that crenarchaea are one of the most abundant forms of life on Earth (1, 2); however, we know essentially nothing about how cell division occurs in these organisms. This is of particular interest because the sequenced genomes of hyperthermophilic crenarchaeotes lack genes for members of the FtsZ/tubulin and MreB/actin superfamilies of cell division proteins (3-6). The near ubiquity of tubulins and actins underscores these proteins' pivotal roles in cell division processes in bacteria, euryarchaea and eukarya. The absence of orthologs of these proteins in the crenarchaea has prompted us to attempt to identify the crenarchaeal cell division machinery, using species of the genus Sulfolobus as a model system. In metazoa, the ESCRT (endosomal sorting complex required for transport) system, in addition to its roles in endosomal trafficking and viral egress (7-10), plays a role in membrane abscission during cytokinesis (11-13). Most hyperthermophilic crenarchaea encode homologs of ESCRT-III components and the ATPase Vps4 (14,15), that could potentially be involved in cell division (figs. S1 to S3). Sulfolobus encodes four ESCRT-III homologs and a single Vps4 homolog. No homologs of components of the ESCRT-0, -I or -II systems are apparent. The

Members of the crenarchaeal kingdom, such as Sulfolobus, divide by binary fission yet lack genes for the otherwise near-ubiquitous tubulin and actin superfamilies of cytoskeletal proteins. Recent work has established that Sulfolobus homologs of the eukaryotic ESCRT-III and Vps4 components of the ESCRT machinery play an important role in Sulfolobus cell division. In eukaryotes, several pathways recruit ESCRT-III proteins to their sites of action. However, the positioning determinants for archaeal ESCRT-III are not known. Here, we identify a protein, CdvA, that is responsible for recruiting Sulfolobus ESCRT-III to membranes. Overexpression of the isolated ESCRT-III domain that interacts with CdvA results in the generation of nucleoid-free cells. Furthermore, CdvA and ESCRT-III synergize to deform archaeal membranes in vitro. The structure of the CdvA/ESCRT-III interface gives insight into the evolution of the more complex and modular eukaryotic ESCRT complex.

The three-dimensional organization of chromosomes can have a profound impact on their replication and expression. The chromosomes of higher eukaryotes possess discrete compartments that are characterized by differing transcriptional activities. Contrastingly, most bacterial chromosomes have simpler organization with local domains, the boundaries of which are influenced by gene expression. Numerous studies have revealed that the higher-order architectures of bacterial and eukaryotic chromosomes are dependent on the actions of structural maintenance of chromosomes (SMC) superfamily protein complexes, in particular, the nearuniversal condensin complex. Intriguingly, however, many archaea, including members of the genus Sulfolobus do not encode canonical condensin. We describe chromosome conformation capture experiments on Sulfolobus species. These reveal the presence of distinct domains along Sulfolobus chromosomes that undergo discrete and specific higherorder interactions, thus defining two compartment types. We observe causal linkages between compartment identity, gene expression, and binding of a hitherto uncharacterized SMC superfamily protein that we term ''coalescin.''