Slpr is related to human mixed lineage kinases among a larger group of JNKKK proteins. (A) Protein sequence alignment of fly MLK (Slpr) with human family members MLK2 (MST) and MLK3 (SPRK) in the N-terminal region encompassing the conserved domains as indicated above the sequence, SH3 (black line), kinase (gray line), LZ (dotted line), CRIB (double line). Residues highlighted in gray mark the sites in which slpr mutations are located. (B) Phylogenetic relationship of Slpr compared with the putative JNKKK protein kinase group. Relationship is based on kinase sequence only, using the CLUSTAL method with PAM250 residue weight. Five of six families are represented in the fly genome as a single homolog compared with human sequences. The MLK family shares ∼60% amino acid identity with Slpr. The kinases with leucine zipper motifs in the LZK branch are ∼36% identical to Slpr. Less related kinases in the JNKKK group are TAKs (∼25% identity), ASKs (∼20% identity), and MEKKs (∼18% identity), which show the greatest sequence divergence.