The Expanding Landscape of Acidophilic Protein Kinases

Acidophilic kinases are serine/threonine protein kinases that preferentially phosphorylate substrates bearing negatively charged determinants near the phosphoacceptor site. These determinants may be encoded by aspartate or glutamate, or introduced by prior phosphorylation of serine, threonine, or tyrosine residues. CK2 is the most representative of this historically small group of kinases because of its high basal activity, broad localization, and extensive substrate repertoire. Kinome-wide specificity profiling covering more than 84% of the active human S/T kinome has now provided a near-comprehensive view of acidophilic kinases across several families. Here, we compare these enzymes through three interconnected features: subcellular localization, substratome size, and consensus-motif specificity. We broadly categorize them as D/E-directed kinases or phosphate-directed/primed kinases, while recognizing that these preferences are not mutually exclusive. We then compare intrinsic preferences measured with peptide libraries with those observed in bona fide substrates. Despite their shared preference for negative charge, acidophilic kinases display markedly different positional requirements for acidic or phosphorylated determinants. For kinases that rely strongly on consensus-sequence recognition, these preferences can generate distinctive kinase-specific fingerprints that can support the attribution of experimentally validated phosphosites, particularly when combined with subcellular localization.