A lipid-binding domain in Wnt: a case of mistaken identity?

Abstract

The alignments of sPLA2s and Wnts of Reichsman et al. [4xSequence homology between Wingless/Wnt-1 and a lipid-binding domain in secreted phospholipase A2. Reichsman, F, Moore, HM, and Cumberledge, S. Curr Biol. 1999; 9: R353–R355Abstract | Full Text | Full Text PDF | PubMedSee all References[4] and Barnes and Russell [17xA lipid-binding domain in Wnt: a case of mistaken identity?. Barnes, MR and Russell, RB. Curr Biol. 1999; 9: R717–R718Abstract | Full Text | Full Text PDF | PubMed | Scopus (1)See all References[17] were generated using two different paradigms. Barnes and Russell compared the physico-chemical properties of the amino acid residues; we used the Gonnet PAM 250 substitution matrix to evaluate amino acid similarities. PAM (percentage of acceptable point mutations) matrices have been widely recommended for detecting related proteins [[18]xSchwartz, RM and Dayhoff, MO. : 353–362See all References, [19]xThe significance of protein sequence similarities. Collins, JF, Coulson, AFW, and Lyall, A. Comp App Biosci. 1988; 4: 67–71PubMedSee all References, [20]xAmino acid substitution matrices from an information theoretic perspective. Altschul, S. J Mol Biol. 1991; 219: 555–565Crossref | PubMed | Scopus (389)See all References]. These substitution matrices are based on evolutionary relationships between proteins and are generated by analyzing point mutations in closely related proteins. Using the PAM matrices, we find that sPLA2 App-K49 residues 9–61 are more similar to the carboxy-terminal portion of Wingless than to the corresponding bovine sPLA2 sequences (44% versus 36% similarity). A total of 24 residues are conserved between sPLA2 and Wingless; 7 of these are cysteines. Note that App-K49 lacks the conserved Ala49, does not bind Ca2+ and has no catalytic activity, yet retains the ability to bind membranes and disrupt phospholipid vesicles [21xThe growing phospholipase A2 superfamily of signal transduction enzymes. Dennis, EA. Trends Biochem Sci. 1997; 22: 1–2Abstract | Full Text PDF | PubMed | Scopus (705)See all References[21].The first 60 residues of sPLA2 form the helix–loop–helix backbone that constitutes much of the lipid-binding pocket. The amino-terminal helix and loop are important for interfacial membrane binding, whereas residues in helix 2 contribute to the active site. As the rest of the App-K49 sequence has no similarity to Wingless, the two protein families probably have very different tertiary structures. Site-directed mutagenesis studies on sPLA2 [22xPhospholipase A2 engineering. The roles of disulfide bonds in structure, conformational stability, and catalytic function. Zho, H, Dupureur, CM, Zhang, X, and Tsai, MD. Biochemistry. 1995; 34: 15307–15314Crossref | PubMedSee all References[22] have shown that of the seven conserved disulphide bonds, only one, Cys29–Cys45, is required for lipase activity. Cys29 and Cys45 correspond to Cys428 and Cys445 in Wingless. These cysteines are conserved in the Wnts, and Cys445 is required for Wingless activity. Nonetheless, we agree that it is difficult to evaluate the significance of these sequence similarities without more information on Wnt function and structure.