Constitutive Activity in an Ancestral Form of Abl Tyrosine Kinase

Abstract

The c-abl proto-oncogene encodes a nonreceptor tyrosine kinase that is found in all metazoans, and is ubiquitously expressed in mammalian tissues. The Abl tyrosine kinase plays important roles in the regulation of mammalian cell physiology. Abl-like kinases have been identified in the genomes of unicellular choanoflagellates, the closest relatives to the Metazoa, and in related unicellular organisms. Here, we have carried out the first characterization of a premetazoan Abl kinase, MbAbl2, from the choanoflagellate Monosiga brevicollis. The enzyme possesses SH3, SH2, and kinase domains in a similar arrangement to its mammalian counterparts, and is an active tyrosine kinase. MbAbl2 lacks the N-terminal myristoylation and cap sequences that are critical regulators of mammalian Abl kinase activity, and we show that MbAbl2 is constitutively active. When expressed in mammalian cells, MbAbl2 strongly phosphorylates cellular proteins on tyrosine, and transforms cells much more potently than mammalian Abl kinase. Thus, MbAbl2 appears to lack the autoinhibitory mechanism that tightly constrains the activity of mammalian Abl kinases, suggesting that this regulatory apparatus arose more recently in metazoan evolution.

Phosphotyrosine signalling and the origin of animal multicellularity

“…Similar results were also observed in MbAbl2 [45] (but probably not in HMTK1 [46]) (electronic supplementary material, figure S1 and table S1), indicating the early establishment of this positive regulatory mechanism in holozoan evolution.…”

“…One of four Src isoforms (EfSrc2) in the sponge Ephydatia fluviatilis also lacks Csk-mediated negative regulation (electronic supplementary material, Considering the inhibitory effects of elevated Src activity on intercellular interactions, the evolution of negative regulation of Src activity (and, more generally, of TK activity) may contribute to the maintenance of intercellular interactions and thereby the origin of animal multicellularity [16,42,43,47,49]. This hypothesis may be further supported by MbAbl2 that exhibits constitutive activity compared to its tightly regulated mammalian counterpart, owing to its lack of N-terminal myristoylation sequence and cap region that regulate mammalian Abl activity [45,54]. Furthermore, this proposed correlation between regulatory evolution and multicellular transition mirrors a model where cancer, which is featured with loss of regulatory constraints that leads to lost intercellular cooperation and unlimited cell proliferation, can be viewed as a reverse evolution from multicellularity to unicellularity [2][3][4].…”

“…Like their mammalian counterparts, all these Src and Csk family kinases share a SH3-SH2-TyrKc domain architecture, with Src having an additional myristoylation sequence at the N-terminus, but MbSrc4 has a unique N-terminal C2 domain instead of the myristoylation sequence [16,19,23,[42][43][44]47]. MbAbl2 possesses the SH3-SH2-TyrKc combination while lacking the N-terminal myristoylation sequence, cap region and the large C-terminal portion that feature the mammalian counterparts [19,45]. In HMTK1, three PTB domains lie directly N-terminal to TyrKc domain, whereas PTB domains are not found in metazoan TKs [19,46].…”

Multifunctional Abl kinases in health and disease

“…The auto-inhibited conformation is further stabilized by an N-terminal cap and binding of the N-terminal myristoylated residue to a hydrophobic pocket within the kinase domain (see poster). Interestingly, the M. brevicollis ABL2 lacks the N-terminal myristoylation and cap sequences, and was reported to be constitutively active when expressed in mammalian cells (Aleem et al, 2015).…”

Choanoflagellate models — Monosiga brevicollis and Salpingoeca rosetta

“…The M. brevicollis genome revealed the presence of 128 TKs [3,21], opposed to 90 in the human genome [25]. Only 7 M. brevicollis TKs are homologous to animal TKs [21], one of which (Abl2) has been shown to phosphorylate cellular proteins when expressed in mammalian cells [26]. The other M. brevicollis TKs show domain combinations distinct from animal TKs [3,21], revealing individual diversification of TKs in choanoflagellates and animals [21,27].…”