Triple Beta Helix as a Molecular Clamp

The P22 tailspike is one of the few complex proteins whose folding intermediates have been identified both in vivo and in vitro. Each subunit of the trimeric tailspike adhesin includes an elongated processive 13 rung parallel ß helix. Kristen Cook and Cammie Haase-Pettingell are characterizing early intermediates in the in vitro refolding and misfolding of the tailspike chains. After residue 540 the three chains wrap around each other forming an interdigitated triple ß-coil. Formation of this domain proceeds through the protrimer intermediate in the folding pathway, in which the chains are associated but not yet interdigitated. Upon completion of braiding and folding, the melting temperature increases by more than 40ºC, and the mature protein acquires its SDS and protease resistance (Kreisberg, Haase-Pettingell et al, 2002). Thus this motif functions as a molecular clamp, conferring high thermostability on the native trimer. Dr. Peter Weigele has isolated a set of amino acid substitutions within the braided sequence forming the clamp, and shown that a number of these block the transition from the protrimer to the native trimer. The sequence appears to both terminate the single ß-helical fold and direct the strands into the triple-beta-helical motif, conferring the leap in thermostability.