Refined Crystal Structure of Rat Parvalbumin, a Mammalian α-lineage Parvalbumin, at 2·0 Å Resolution

Abstract

We present here the X-ray crystal structure of the rat α-parvalbumin from fast twitch muscle. This protein ( M r 11·8kDa) crystallizes in space group P2 12 12 1 with unit cell dimensions of a=34·3 Å, b=55·0 Å, c=156·1 Å and three molecules in the asymmetric unit. The protein structure was solved by the molecular replacement method and has been refined to a crystallographic R-factor ( R=Σ∥ F o|-| F c∥/Σ| F o|)) of 0·181 for all reflections with I/σ(I)(l) ≥ 2 ( I=intensity) between 8·0 and 2·0 Å resolution. The molecules located most easily in the molecular replacement rotation function had lower overall thermal motion parameters and higher numbers of intermolecular crystal packing contacts. The overall fold of the polypeptide chain for the rat α-parvalbumin is similar to other known parvalbumin structures (root-mean-square deviations in α-carbon atom positions range from 0·60 to 0·87 Å). There are two Ca 2+-binding sites in parvalbumins, and them is some evidence for a third ion-binding site, adjacent to the CD site, in the rat species. The level of structural variability among the best-ordered regions of the three independent rat, α-parvalbumin molecules in the crystallographic asymmetric unit, is two to three times higher than the mean coordinate error (0·10 Å), indicating flexibility in the molecule. Sequence differences between α and β-lineage parvalbumins result in repacking of the hydrophobic core and some shifts in the protein backbone. The shifts are localized, however, and entire helices do not shift as rigid units.