ORIGIN OF THE NEFA AND NUC SIGNAL SEQUENCES

The human protein NEFA binds calcium, contains a leucine zipper repeat that does not form a homodimer, and is proposed (along with the homol ogous Nuc protein) to have a common evolutionary history with an EF-ha nd ancestor. We have isolated and characterized the N-terminal domain of NEFA that contains a signal sequence inferred from both endoprotein ase Asp-N (Asp-N) and tryptic digests. Analysis of this N-terminal seq uence shows significant similarity to the conserved multiple domains o f the mitochondrial carrier family (MCF) proteins. The leader sequence of Nuc is, however, most similar to the signal sequences of membrane and/or secreted proteins (e.g., mouse insulin-like growth factor recep tor). We suggest that the divergent NEFA and Nuc N-terminal sequences may have independent origins and that the common high hydrophobicity g overns their targeting to the ER. These results provide insights into signal sequence evolution and the multiple origins of protein targetin g.