In This Issue

1173 Direct identification of the site of binding on the chaperone SecB for the amino terminus of the translocon motor SecA

Linda L. Randall and Michael T. Henzl

Protein export mediated by the general secretory Sec system in Escherichia coli proceeds by a dynamic transfer of a precursor polypeptide from the chaperone SecB to the SecA ATPase motor of the translocon and subsequently through the membrane-embedded SecYEG channel. The complex of SecA and SecB is stabilized by several separate sites of contact. Here, the authors demonstrate directly an interaction between the N-terminal 11 residues of SecA and the C-terminal 13 residues of SecB by isothermal titration calorimetry and analytical sedimentation velocity centrifugation. We discuss the unusual binding properties of SecA and SecB in context of a model for transfer of the precursor along the pathway of export.

1137 Structure and function of enzymes in heme biosynthesis

Gunhild Layer, Joachim Reichelt, Dieter Jahn, and Dirk W. Heinz

The red color of blood is caused by the heme cofactor of the oxygen carrier hemoglobin. Hemes are also of fundamental importance for electron transport during energy generating processes including respiration and photosynthesis. The biosynthesis of heme is performed by an array of exciting enzymes with unique catalytic features. The complete set of enzymes has been structurally elucidated providing the structural basis of catalysis and mutations leading to human disease. In this review, the structure-function relationship of all heme biosynthesis enzymes characterized thus far is summarized.

1243 Purification of Escherichia coli RNA polymerase using a self-cleaving elastin-like polypeptide tag

Baley A. Fong, Alison R. Gillies, Iraj Ghazi, Gary LeRoy, Kathleen C. Lee, Lars F. Westblade, and David W. Wood

Many important proteins exist in an active form as multi-subunit complexes, which must be purified in a native state for effective study. One of these is the E. coli RNA polymerase complex, consisting of a five-subunit core enzyme that can be associated with a variety of cofactors. By tagging one of the core subunits with a reversibly precipitating self-cleaving tag, the entire complex can be purified through a relatively straightforward series of precipitation and resuspension steps. This method shows promise for the purification of other multi-subunit complexes, and may eventually be useful for identifying other types of protein-protein interactions.

1127 Physical-chemical determinants of coil conformations in globular proteins

Lauren L. Perskie and George D. Rose

Physical-chemical determinants of coil conformations in globular proteins. Monte Carlo simulations using remarkably simple energy terms (sterics and hydrogen-bonding) successfully capture the conformations of short turn-like segments (≤4 residues) in the protein coil library (www.roselab.jhu.edu/coil). Most longer coil segments (>90%), ranging from 5-20 residues, are structural composites of these shorter primitives. Eight common primitives are shown here. Segments with φ,ψ-angles that are primarily α-helical, extended, polyproline II or αL are colored red, blue, green and yellow, respectively. Segments with mixed φ,ψ-angles are indicated by mixtures of these four colors.