Gabriela Casanova-Sepúlveda probes PDZ domain of LIM Kinases involved in cytoskeleton remodeling
Cytoskeleton remodeling exists as a fundamental part of cellular processes, allowing cells to take on dynamic forms for essential events, including proliferation, cytokinesis, and differentiation. A key part of cytoskeleton remodeling is actin severing, a process regulated by LIM domain kinases (LIMK). These kinases function by phosphorylating the actin depolymerizing factor cofilin with high-fidelity recognition, resulting in actin-severing suppression. LIM kinases are highly important in the cascade that results in actin remodeling, yet many details about LIMK function are unknown.
MB&B graduate student, Gabriela Casanova-Sepúlveda, from the Boggon lab explored the mechanism of autoregulation in the two mammalian LIMKs: LIMK1 and LIMK2. These proteins both contain an N-terminal PDZ domain upstream of the catalytic kinase domain. While early studies suggested the that the N-terminal domains played a role in regulation of catalytic activity, it remained unclear how the PDZ domain impacted LIMK autoregulation. To help answer this question, she studied LIMK PDZ domain and provided an in-depth analysis of the 2.0 Å crystal structure of the LIMK2 PDZ domain. First, they found a canonical PDZ fold that had a unique divergent feature: the presence of an arginine in the x-Φ-G-Φ motif (where x represents any, and Φ represents hydrophobic amino acid). In canonical PDZ domains the x-Φ-G-Φ motif is important for C-terminal peptide recognition from protein partners. In the case of LIMK PDZ domain, this study found that this charged residue is oriented towards the hydrophobic core of the domain and promotes the creation of a shallow peptide binding cleft.
The second important finding was that LIMK PDZ domain contains a highly conserved surface distal to the canonical peptide binding cleft. Mutation of this conserved region disrupted the regulation of the LIM kinase, suggesting this domain plays a direct role in auto inhibition of kinase activity. This conserved site suggests that multiple independent, or partially independent, steps are required to fully activate the LIM domain kinases.
In all, the study was able to elucidate important molecular-level features of the LIMK PDZ domain and its role in regulation of protein kinase activity. The authors, from the Boggon and Turk labs, are excited to share these findings and hopes this helps increase the understanding of protein kinase autoregulation by PDZ domains. The full article, published in Nature Communications, can be found here