Mutations in the Leucine-rich repeat kinase-2 (LRRK2) gene have been associated with familial and sporadic Parkinson’s disease (PD), which is the second most common neurodegenerative disorder worldwide after Alzheimer’s. LRRK2 is a multi-domain protein that contains an LRR (leucine rich repeats) motif, a COR (C-terminal of Ras of Complex) domain, a WD40 (Trp-Asp 40) motif and two distinct functional domains; a GTPase domain and a protein kinase domain. The majority of pathological mutations in LRRK2 are clustered within the three domains that form the enzymatic core. The most prevalent mutant form of LRRK2 is the Gly2019Ser mutation, which up-regulates LRRK2 kinase activity. Gly2019 is located within the Asp-Tyr-Gly-Mg2+-binding motif of the kinase domain.
Moesin was identified as a physiological substrate of LRRK2 in a kinase substrate tracking and elucidation (KESTREL) screen using rat brain extracts. Belonging to the ezrin/redixin/moesin ERM family of proteins, Moesin functions as an anchor between actin-based cytoskeletons and plasma membranes. Analysis of radiolabelled phosphopeptides, derived from LRRK2 phosphorylated Moesin, mapped Thr558 as a phosphorylation site. LRRK2 was also shown to phosphorylate ezrin (Thr567) and radixin (Thr564) at residues equivalent to Moesin Thr558; consequently, the LRRKtide peptide (RLGRDKYKTLRQIRQ) from Moesin that encompasses the Thr558 was derived. Although LRRKtide contains both threonine and tyrosine residues, LRRK2 acts on LRRKtide predominately as a serine/threonine kinase and not as a tyrosine kinase.
LRRKtide is widely used for the enzymatic characterization of LRRK2 kinase activity and inhibition studies and is considered to be a more efficient substrate for quantitative assays, compared to measuring LRRK2 auto-phosphorylation. Multiple studies have used LRRKtide to assess the effects of pathogenic mutations on LRRK2 kinase activity and as LRRK2 is clinically linked to PD, LRRK2 kinase has been considered as a therapeutic target. In addition, LRRKtide has been used for comparing the efficiencies of Mg2+ and Mn2+ divalent metal ions as ATP cofactors, to support LRRK2 kinase activity.
Novel LRRK2 kinase inhibitors have been identified and evaluated from a library of compounds using high-throughput screening that detects LRRKtide phosphorylation using mass spectrometry. The diverse use of LRRKtide has contributed to a better understanding of the role of LRRK2 in Parkinson’s Disease, and provides further possibilities for future investigations into combating the neurodegenerative disorder.