The agony column

It has recently been shown that a class of secreted proteins — the Dickkopfs — are released by the Spemann organizer of Xenopus and are potent antagonists of Wnt signaling [1]. Although it has been reported that the Dickkopf (Dkk) proteins contain two cysteine-rich domains, no direct functional or structural conclusions have been drawn from their sequences. The role of Wnt signaling in animal development has recently become clearer, with the discovery of new components of this pathway [2], and is particularly well illustrated by experiments involving Xenopus head formation by the Spemann organizer [2] and wing development in Drosophila [3]. These studies led to the characterization of the complete signaling pathway that is initiated extracellularly by the binding of Wnt to the Frizzled-like seven-transmembrane receptors [2], and proceeds intracellularly through a G-protein-coupled cascade [4] and the Armadillo and Lef proteins, which function in the nucleus by regulating transcription [5]. This signaling cascade is necessary in vertebrate development to initiate posterior structures and is antagonized by molecules such as Dkk and Frzb, which are secreted by the Spemann organizer and are required for head induction [6]. To gain a better understanding of the role of Dkks in Wnt signaling, we carried out a detailed analysis of Dkk sequences and show here that they have a colipase fold. This observation may have direct implications for the function of the Dkks in signaling. It has been noted that the Dkks have two cysteine-rich domains ([6]; see also below), and we found additionally that the carboxy-terminal domain has the pattern of cysteines typical of the colipase domain (Figure 1). An iterative database search of the non-redundant protein database at the National Center for Biotechnology Information (NCBI) using the PSI-BLAST program [7], with the Dkk sequences as queries, recovered the colipases from different mammals at a statistically significant expectation (e) value of 10–4 in the second iteration. This was confirmed by reverse searches using different colipases, which retrieved the Dkks at similar e values within three iterations. Multiple alignment based secondary structure prediction using the PHD program [8] and subsequent threading [9] through the Magazine R477