Figure 1. Domain architecture of metacaspase Mca1.

The annotated metacaspase Mca1 consists of 432 amino acids (Mca1₄₃₂), whereas previously two upstream translational start sites (57 and 63 base pairs upstream) have been annotated that give rise to Mca1 proteins of 451 amino acids (Mca1₄₅₁, N-terminal extension of MSLEVYLNYHQRRPTRFTI) and 453 amino acids length (Mca1₄₅₃, N-terminal extension of MKMSLEVYLNYHQRRPTRFTI). In this study, we additionally used a 454 amino acids long Mca1 protein (Mca1₄₅₄, N-terminal extension of MGKMSLEVYLNYHQRRPTRFTI) that introduced an additional glycine residue after the methionine because of cloning considerations as outlined in the text and Materials and Methods. The additional N-terminal amino acids of the long Mca1 variants display a hydrophilic-hydrophobic charge distribution as shown in the upper left part of the figure. The hydrophobicity of the MGKMSLEVYLNYHQRRPTRFTIpeptide stretch was calculated using the ProtScale tool of the ExPASy Proteomics Server (www.expasy.org) that uses an amino acid scale described by Kyte and Doolittle [28]. All Mca1 variants also contain a Q/N-rich region reminiscent of the prion-forming domain (PFD) of Sup35 and other yeast prion proteins. Additionally, the putative PFD of Mca1 consists of several imperfect repeats of QQYGas visualized in the figure.