title: Non-native alpha-helix formation is not necessary for folding of lipocalin: comparison of burst-phase folding between tear lipocalin and beta-lactoglobulin.
abstract: Tear lipocalin and beta-lactoglobulin are members of the lipocalin superfamily. They have similar tertiary structures but unusually low overall sequence similarity. Non-native helical structures are formed during the early stage of beta-lactoglobulin folding. To address whether the non-native helix formation is found in the folding of other lipocalin superfamily proteins, the folding kinetics of a tear lipocalin variant were investigated by stopped-flow methods measuring the time-dependent changes in circular dichroism (CD) spectrum and small-angle X-ray scattering (SAXS). CD spectrum showed that extensive secondary structures are not formed during a burst-phase (within a measurement dead time). The SAXS data showed that the radius of gyration becomes much smaller than in the unfolded state during the burst-phase, indicating that the molecule is collapsed during an early stage of folding. Therefore, non-native helix formation is not general for folding of all lipocalin family members. The non-native helix content in the burst-phase folding appears to depend on helical propensities of the amino acid sequence.
authors.affiliation: Department of Bioinformatics, Soka University, Hachioji, Tokyo 192-8577, Japan.
authors.forename: Seiichi
authors.initials: S
authors.lastname: Tsukamoto
authors.forename: Takako
authors.initials: T
authors.lastname: Yamashita
authors.forename: Yoshiteru
authors.initials: Y
authors.lastname: Yamada
authors.forename: Kazuo
authors.initials: K
authors.lastname: Fujiwara
authors.forename: Kosuke
authors.initials: K
authors.lastname: Maki
authors.forename: Kunihiro
authors.initials: K
authors.lastname: Kuwajima
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Matsumura
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kihara
authors.forename: Hideaki
authors.initials: H
authors.lastname: Tsuge
authors.forename: Masamichi
authors.initials: M
authors.lastname: Ikeguchi
mesh_terms: D000595:Amino Acid Sequence; D002942:Circular Dichroism; D004926:Escherichia coli / Q000235:genetics; D006801:Humans; D007700:Kinetics; D007782:Lactoglobulins / Q000737:chemistry / Q000378:metabolism*; D054835:Lipocalin 1 / Q000737:chemistry / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism*; D008969:Molecular Sequence Data; D017354:Point Mutation; D011489:Protein Denaturation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D053838:Scattering, Small Angle; D016415:Sequence Alignment; D014508:Urea / Q000378:metabolism; D014961:X-Ray Diffraction
chemical_list: D007782:Lactoglobulins; D054835:Lipocalin 1; D014508:Urea