Archaeal surface layer proteins contain β propeller, PKD, and β helix domains and are related to metazoan cell surface proteins

The surface layer of archaeobacteria protects cells from extreme environments and, in Methanosarcina, may regulate cell adhesion. We identify three domain types that account for the complete architecture of numerous Methanosarcina surface layer proteins (SLPs). We solve the crystal structure for two of these domains, which correspond to the two N-terminal domains of an M. mazei SLP. One domain displays a unique, highly symmetrical, seven-bladed beta propeller fold, and the other belongs to the polycystic kidney disease (PKD) superfamily fold. The third domain is predicted to adopt a beta helix fold. These domains have homologs in metazoan cell surface proteins, suggesting remarkable relationships between domains in archaeal SLPs and metazoan cell surface proteins.