This is the conserved leucine-rich domain (LRD) within the C-terminal domain of SPIN90 (also known as NCK interacting protein with SH3 domain (NCKIPSD)) from animals, Ldb17 from budding yeasts and Dip1 from fission yeasts [1]. Ldb17 is involved in the regulation of endocytosis in yeast [2]. SPIN90 is an F-actin binding protein that regulates actin polymerization and endocytosis [3]. It associates with and activates the Arp2/3 complex, without requiring a preformed actin filament [2,4,5]. This LRD, within the C-terminal armadillo repeat (ARM) domain of SPIN90, is important for activation of Arp2/3 complex [1].

1: Structure of the nucleation-promoting factor SPIN90 bound to the actin filament nucleator Arp2/3 complex. Luan Q, Liu SL, Helgeson LA, Nolen BJ; EMBO J. 2018; [Epub ahead of print] PMID:30322896

2: Regulators of yeast endocytosis identified by systematic quantitative analysis. Burston HE, Maldonado-Baez L, Davey M, Montpetit B, Schluter C, Wendland B, Conibear E; J Cell Biol. 2009;185:1097-1110. PMID:19506040

3: F-actin binding region of SPIN90 C-terminus is essential for actin polymerization and lamellipodia formation. Kim DJ, Kim SH, Kim SM, Bae JI, Ahnn J, Song WK; Cell Commun Adhes. 2007;14:33-43. PMID:17453829

4: Interaction of SPIN90 with the Arp2/3 complex mediates lamellipodia and actin comet tail formation. Kim DJ, Kim SH, Lim CS, Choi KY, Park CS, Sung BH, Yeo MG, Chang S, Kim JK, Song WK; J Biol Chem. 2006;281:617-625. PMID:16253999

5: A novel neural Wiskott-Aldrich syndrome protein (N-WASP) binding protein, WISH, induces Arp2/3 complex activation independent of Cdc42. Fukuoka M, Suetsugu S, Miki H, Fukami K, Endo T, Takenawa T; J Cell Biol. 2001;152:471-482. PMID:11157975