Constitutive Activity in an Ancestral Form of Abl Tyrosine Kinase

A stress-responsive p38 signaling axis in choanoflagellates

Animal kinases regulate cellular responses to environmental stimuli, including cell differentiation, migration, survival, and response to stress, but the ancestry of these functions is poorly understood. Choanoflagellates, the closest living relatives of animals, encode homologs of diverse animal kinases and have emerged as model organisms for reconstructing animal origins. However, efforts to identify key kinase regulators in choanoflagellates have been constrained by the limitations of currently available genetic tools. Here, we report on a framework that combines small molecule-driven kinase discovery with targeted genetics to reveal kinase function in choanoflagellates. To study the physiological roles of choanoflagellate kinases, we established two high-throughput platforms to screen the model choanoflagellate Salpingoeca rosetta with a curated library of human kinase inhibitors. We identified 95 diverse kinase inhibitors that disrupt S. rosetta cell proliferation. By focusing on one inhibitor, sorafenib, we identified a p38 kinase as a regulator of the heat shock response in S. rosetta. This finding reveals a conserved p38 function between choanoflagellates, animals, and fungi. Moreover, this study demonstrates that existing kinase inhibitors can serve as powerful tools to examine the ancestral roles of kinases that regulate modern animal development.

Evaluation of residue variability in a conformation-specific context and during evolutionary sequence reconstruction narrows drug resistance selection in Abl1 tyrosine kinase

Diseases with readily available therapies may eventually prevail against the specific treatment by the acquisition of resistance. The constitutively active Abl1 tyrosine kinase known to cause chronic myeloid leukemia is an example, where patients may experience relapse after small inhibitor drug treatment. Mutations in the Abl1 tyrosine kinase domain (Abl1-KD) are a critical source of resistance and their emergence depends on the conformational states that have been observed experimentally: the inactive state, the active state, and the intermediate inactive state that resembles Src kinase. Understanding how resistant positions and amino acid identities are determined by selection pressure during drug treatment is necessary to improve future drug development or treatment decisions. We carry out in silico site-saturation mutagenesis over the Abl1-KD structure in a conformational context to evaluate the in situ and conformational stability energy upon mutation. Out of the 11 studied resistant positions, we determined that 7 of the resistant mutations favored the active conformation of Abl1-KD with respect to the inactive state. When, instead, the sequence optimization was modeled simultaneously at resistant positions, we recovered five known resistant mutations in the active conformation. These results suggested that the Abl1 resistance mechanism targeted substitutions that favored the active conformation. Further sequence variability, explored by ancestral reconstruction in Abl1-KD, showed that neutral genetic drift, with respect to amino acid variability, was specifically diminished in the resistant positions. Since resistant mutations are susceptible to chance with a certain probability of fixation, combining methodologies outlined here may narrow and limit the available sequence space for resistance to emerge, resulting in more robust therapeutic treatments over time.

MotifAnalyzer-PDZ: A computational program to investigate the evolution of PDZ-binding target specificity

Recognition of short linear motifs (SLiMs) or peptides by proteins is an important component of many cellular processes. However, due to limited and degenerate binding motifs, prediction of cellular targets is challenging. In addition, many of these interactions are transient and of relatively low affinity. Here, we focus on one of the largest families of SLiM-binding domains in the human proteome, the PDZ domain. These domains bind the extreme C-terminus of target proteins, and are involved in many signaling and trafficking pathways. To predict endogenous targets of PDZ domains, we developed MotifAnalyzer-PDZ, a program that filters and compares all motif-satisfying sequences in any publicly available proteome. This approach enables us to determine possible PDZ binding targets in humans and other organisms. Using this program, we predicted and biochemically tested novel human PDZ targets by looking for strong sequence conservation in evolution. We also identified three C-terminal sequences in choanoflagellates that bind a choanoflagellate PDZ domain, the Monsiga brevicollis SHANK1 PDZ domain (mbSHANK1), with endogenously-relevant affinities, despite a lack of conservation with the targets of a homologous human PDZ domain, SHANK1. All three are predicted to be signaling proteins, with strong sequence homology to cytosolic and receptor tyrosine kinases. Finally, we analyzed and compared the positional amino acid enrichments in PDZ motif-satisfying sequences from over a dozen organisms. Overall, MotifAnalyzer-PDZ is a versatile program to investigate potential PDZ interactions. This proof-of-concept work is poised to enable similar types of analyses for other SLiM-binding domains (e.g., MotifAnalyzer-Kinase). MotifAnalyzer-PDZ is available at http://motifAnalyzerPDZ.cs.wwu.edu.

The Src module: an ancient scaffold in the evolution of cytoplasmic tyrosine kinases

Tyrosine kinases were first discovered as the protein products of viral oncogenes. We now know that this large family of metazoan enzymes includes nearly one hundred structurally diverse members. Tyrosine kinases are broadly classified into two groups: the transmembrane receptor tyrosine kinases, which sense extracellular stimuli, and the cytoplasmic tyrosine kinases, which contain modular ligand-binding domains and propagate intracellular signals. Several families of cytoplasmic tyrosine kinases have in common a core architecture, the "Src module," composed of a Src-homology 3 (SH3) domain, a Src-homology 2 (SH2) domain, and a kinase domain. Each of these families is defined by additional elaborations on this core architecture. Structural, functional, and evolutionary studies have revealed a unifying set of principles underlying the activity and regulation of tyrosine kinases built on the Src module. The discovery of these conserved properties has shaped our knowledge of the workings of protein kinases in general, and it has had important implications for our understanding of kinase dysregulation in disease and the development of effective kinase-targeted therapies.

Src signaling in a low-complexity unicellular kinome

Creolimax fragrantissima is a member of the ichthyosporean clade, the earliest branching holozoan lineage. The kinome of Creolimax is markedly reduced as compared to those of metazoans. In particular, Creolimax possesses a single non-receptor tyrosine kinase: CfrSrc, the homolog of c-Src kinase. CfrSrc is an active tyrosine kinase, and it is expressed throughout the lifecycle of Creolimax. In animal cells, the regulatory mechanism for Src involves tyrosine phosphorylation at a C-terminal site by Csk kinase. The lack of Csk in Creolimax suggests that a different mode of negative regulation must exist for CfrSrc. We demonstrate that CfrPTP-3, one of the 7 tyrosine-specific phosphatases (PTPs) in Creolimax, suppresses CfrSrc activity in vitro and in vivo. Transcript levels of CfrPTP-3 and two other PTPs are significantly higher than that of CfrSrc in the motile amoeboid and sessile multinucleate stages of the Creolimax life cycle. Thus, in the context of a highly reduced kinome, a pre-existing PTP may have been co-opted for the role of Src regulation. Creolimax represents a unique model system to study the adaptation of tyrosine kinase signaling and regulatory mechanisms.

Phosphotyrosine signalling and the origin of animal multicellularity

The evolution of multicellular animals (i.e. metazoans) from a unicellular ancestor is one of the most important yet least understood evolutionary transitions. Historically, given its indispensable functions in intercellular communication and exclusive presence in metazoans, phosphotyrosine (pTyr) signalling was considered a metazoan-specific evolutionary innovation that might have contributed to the origin of metazoan multicellularity. However, recent studies have led to a new understanding of pTyr signalling evolution and its role in the metazoan origin. Sequence analyses have unravelled a much earlier emergence of pTyr signalling in eukaryotic evolution. Even so, several distinct properties of holozoan pTyr signalling may have paved the way for a hypothesized functional transition of pTyr signalling at the multicellular origin, from environmental sensing to intercellular communication, and for it to evolve as a powerful intercellular signalling system for multicellularity. Biochemical analyses of premetazoan pTyr signalling components have further revealed the premetazoan origin of many key features of metazoan pTyr signalling, and the metazoan establishment of others, including the Csk-mediated negative regulation of the activity of Src, a conserved tyrosine kinase in the Holozoa. Finally, potential future directions are discussed, with a stress on the biological functions of premetazoan pTyr signalling via newly developed gene manipulation tools in non-animal holozoans.

Multifunctional Abl kinases in health and disease

The Abelson tyrosine kinases were initially identified as drivers of leukemia in mice and humans. The Abl family kinases Abl1 and Abl2 regulate diverse cellular processes during development and normal homeostasis, and their functions are subverted during inflammation, cancer and other pathologies. Abl kinases can be activated by multiple stimuli leading to cytoskeletal reorganization required for cell morphogenesis, motility, adhesion and polarity. Depending on the cellular context, Abl kinases regulate cell survival and proliferation. Emerging data support important roles for Abl kinases in pathologies linked to inflammation. Among these are neurodegenerative diseases and inflammatory pathologies. Unexpectedly, Abl kinases have also been identified as important players in mammalian host cells during microbial pathogenesis. Thus, the use of Abl kinase inhibitors might prove to be effective in the treatment of pathologies beyond leukemia and solid tumors. In this Cell Science at a Glance article and in the accompanying poster, we highlight the emerging roles of Abl kinases in the regulation of cellular processes in normal cells and diverse pathologies ranging from cancer to microbial pathogenesis.

Choanoflagellate models - Monosiga brevicollis and Salpingoeca rosetta

Choanoflagellates are the closest single-celled relatives of animals and provide fascinating insights into developmental processes in animals. Two species, the choanoflagellates Monosiga brevicollis and Salpingoeca rosetta are emerging as promising model organisms to reveal the evolutionary origin of key animal innovations. In this review, we highlight how choanoflagellates are used to study the origin of multicellularity in animals. The newly available genomic resources and functional techniques provide important insights into the function of choanoflagellate pre- and postsynaptic proteins, cell-cell adhesion and signaling molecules and the evolution of animal filopodia and thus underscore the relevance of choanoflagellate models for evolutionary biology, neurobiology and cell biology research.