Novel protein structural motifs containing two-turn and longer 3(10)-helices

The 3(10)-helix constitutes a small but significant fraction of secondary s tructural elements in proteins. Protein data base surveys have shown these helices to be present as a-helical extensions, in loops and as connectors b etween beta-strands, The present work focuses on two-turn and longer 3(10)- helices where we establish that two-turn and longer 3(10) helices, unlike t he more abundant single-turn 3(10)-helices, frequently occur independent of any other contiguous secondary Structural elements. More importantly, a la rge fraction of these independent two-turn and longer 3(10)-helices, along with alpha-helices and beta-strands, are found to form novel super-secondar y structural motifs in several proteins With possible implications for prot ein folding, local conformational relaxation and biological functions.