CRYSTAL-STRUCTURE OF THE PHOSPHATIDYLETHANOLAMINE-BINDING PROTEIN FROM BOVINE BRAIN - A NOVEL STRUCTURAL CLASS OF PHOSPHOLIPID-BINDING PROTEINS

Background: Phosphatidylethanolamine-binding protein (PEBP) is a basic protein found in numerous tissues from a wide range of species. The s creening of gene and protein data banks defines a family of PEEP-relat ed proteins that are present in a variety of organisms, including Dros ophila and inferior eukaryotes. PEEP binds to phosphatidylethanolamine and nucleotides in vitro, but its biological function in vivo is not yet known. The expression of PEEP and related proteins seems to be cor related with development and cell morphogenesis, however. To obtain ne w insights into the PEEP family and its potential functions, we initia ted a crystallographic study of bovine brain PEPB. Results: The X-ray crystal structure of bovine brain PEEP has been solved using multiple isomorphous replacement methods, and refined to 1.84 Angstrom resoluti on. The structure displays a beta fold and exhibits one nonprolyl cis peptide bond. Analysis of cavities within the structure and sequence a lignments were used to identify a putative ligand-binding site. This b inding site is defined by residues of the C-terminal helix and the res idues His85, Asp69, Gly109 and Tyr119. This site also corresponds to t he binding site of phosphorylethanolamine, the polar head group of pho sphatidylethanolamine. Conclusions: This study shows that PEEP is not related to the G-protein family nor to known lipid-binding proteins, a nd therefore defines a novel structural family of phospholipid-binding proteins. The PEEP structure contains no internal hydrophobic pocket, as described for lipocalins or small phospholipid-transfer proteins. Nevertheless, in PEEP, a small cavity close to the protein surface has a high affinity for anions, such as phosphate and acetate, and also p hosphorylethanolamine. We suggest that this cavity corresponds to the binding site of the polar head group of phosphatidylethanolamine.