title: Procollagen triple helix assembly: an unconventional chaperone-assisted folding paradigm.
abstract: Fibers composed of type I collagen triple helices form the organic scaffold of bone and many other tissues, yet the energetically preferred conformation of type I collagen at body temperature is a random coil. In fibers, the triple helix is stabilized by neighbors, but how does it fold? The observations reported here reveal surprising features that may represent a new paradigm for folding of marginally stable proteins. We find that human procollagen triple helix spontaneously folds into its native conformation at 30-34 degrees C but not at higher temperatures, even in an environment emulating Endoplasmic Reticulum (ER). ER-like molecular crowding by nonspecific proteins does not affect triple helix folding or aggregation of unfolded chains. Common ER chaperones may prevent aggregation and misfolding of procollagen C-propeptide in their traditional role of binding unfolded polypeptide chains. However, such binding only further destabilizes the triple helix. We argue that folding of the triple helix requires stabilization by preferential binding of chaperones to its folded, native conformation. Based on the triple helix folding temperature measured here and published binding constants, we deduce that HSP47 is likely to do just that. It takes over 20 HSP47 molecules to stabilize a single triple helix at body temperature. The required 50-200 microM concentration of free HSP47 is not unusual for heat-shock chaperones in ER, but it is 100 times higher than used in reported in vitro experiments, which did not reveal such stabilization.
authors.affiliation: Section on Physical Biochemistry, Department of Health and Human Services, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland, United States of America.
authors.forename: Elena
authors.initials: E
authors.lastname: Makareeva
authors.forename: Sergey
authors.initials: S
authors.lastname: Leikin
mesh_terms: D001671:Biochemistry / Q000379:methods; D002152:Calorimetry, Differential Scanning; D002942:Circular Dichroism; D050958:HSP47 Heat-Shock Proteins / Q000737:chemistry; D006801:Humans; D007477:Ions; D018832:Molecular Chaperones / Q000737:chemistry*; D010434:Pepsin A / Q000737:chemistry; D011347:Procollagen / Q000737:chemistry*; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D013696:Temperature; D013816:Thermodynamics
chemical_list: D050958:HSP47 Heat-Shock Proteins; D007477:Ions; D018832:Molecular Chaperones; D011347:Procollagen; D010434:Pepsin A