Membrane Targeting of the N-Terminal Ubiqutin-Like Domain of Kindlin-2 is Crucial for its Regulation of Integrin Activation

Abstract

Kindlin is a three-member subfamily of FERM (four-point-one, ezrin, radixin, moesin) containing proteins that dramatically enhance talin-mediated integrin alphaIIbbeta3 activation, supporting their role as co-activators of integrins. Among them, kindling-2 is widely expressed and highly concentrated at integrin-rich cell-ECM adhesions. By binding to integrin beta cytoplasmic tail via its C-terminal FERM-like domain, kindlin-2 promotes integrin activation. Intriguingly, this activation process depends on the N-terminus of kindlin-2 (K2-N) which precedes the FERM domain. The molecular function of K2-N is unclear. Here we present the solution structure of K2-N, which displays a ubiquitin fold similar to that observed in kindlin-1. Using chemical shift mapping and mutagenesis, we found that K2-N contains a conserved positively charged surface that binds to membrane enriched with highly negatively-charged lipids. We show that while wild-type kindlin-2 is capable of promoting integrin activation, such ability was significantly reduced for its membrane-binding defective mutant. These data suggest a membrane-binding function of the ubiquitin-like domain of kindlin-2, which is likely common for all kindlins to promote their localization to the plasma membrane and control integrin activation.