Examples of protein domains classified by CATH

Some of the architectures new to CATH since 1997:

From: Nucleic Acids Res. 2009 January; 37(Database issue): D310–D314.

“CATH (class, architecture, topology, homology) is a hierarchical protein domain classification (1) where domains are classified manually by curators, guided by prediction algorithms (such as structure comparison). Each protein structure is decomposed into one or more chains which in turn are split into one or more domains before being classified into homologous superfamilies according to both structure and function. At the Class, or C-level, the domains are classified simply on the basis of their secondary structure content [whether they are mostly α-helical (Class 1) or β-sheet (Class 2), contain a significant percentage of both secondary structure elements (Class 3) or contain very little secondary structure (Class 4)]. The domains within each class are then sorted according to their architecture—that is similarities in the arrangements of secondary structures in 3D space. Each architecture (A-level) is further broken down into one or more topology, or fold, groups (T-level), where the connectivity between these secondary structures are taken into account. The domains are then classified into their respective homologous superfamilies (H-level) according to similarities in sequence, structure and/or function. Clustering performed at the H-level (>35% sequence identity and above) then produces one or more sequence families for each of the homologous superfamilies (S-level).”