Proposed molten globule intermediates in fd phage penetration and assembly

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Abstract
The fd filamentous phage can be contracted to short rods called I‐forms and to spheroidal particles called S‐forms. The conversions from fd→I‐forms→S‐forms were previously suggested to mimic steps in fd penetration. The same conversions, in reverse order, were suggested to mimic steps in fd assembly. The I‐forms and S‐forms bind the hydrophobic probe, 1‐anilino‐napthalene‐8‐sulionate (ANS); under the same conditions, fd binds this probe very poorly. Rigidly packed side chains in fd and nonrigidly packed side chains in I‐forms and S‐forms would explain the differences in ANS binding. A compilation of the properties of I‐forms and S‐forms indicate that: (i) they have compact structures; (ii) they have secondary structures of the same type as native phage; (iii) they have non‐native morphologies; and (iv) they may have nonrigid side chain packing. These are the properties of molten globules.