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Center for Nonlinear Studies |
The alpha-actinins (ACTNs) are a highly conserved family of actin-crosslinking proteins in eukaryotes that are critical for various biological processes such as cell motility. Regulation of actin binding by ACTNs has so far been found to occur via several external cues, including epidermal growth factor (EGF)-induced tyrosine phosphorylation, directed proteolysis by calpain enzymes, and binding of calcium ions or phosphoinositide moieties. However, the molecular mechanisms by which these cues regulate ACTN function are still not completely understood, mainly due to the lack of a high-resolution structure that brings together the multiple ACTN domains. Understanding these mechanisms should provide insights into how the modulation of actin cytoskeletal remodeling gives rise to complex cellular phenomena. To this end, we have investigated how various external cues regulate the actin-binding function of human ACTN4, a non-muscle isoform that has been implicated in cancer metastasis. First, we developed and validated a full atomic model of the multidomain assembly that makes up the full ACTN4 homodimer. Our structural model indicates that a novel ternary complex between the CH2, neck, and CaM2 domains comprises the core of this assembly. Next, we recently showed that EGF-induced regulation of ACTN4 function occurs through a novel tandem phosphorylation mechanism in its disordered N-terminal region, where phosphorylation of the functional Y31 site requires prior phosphorylation at the switch Y4 site. This tandem mechanism works in conjunction with m-calpain cleavage of the N-terminal region to generate varied actin-binding responses at the front and rear ends of the cell during motility. In addition to studying how other external cues regulate ACTN4 function, we are also currently developing a network-level model that can help in predicting the actin-binding response of ACTN4 in the presence of multiple cues.
Host: Jeffrey Hyman